A lot of entries use minecrafts builtin predicates like item, block ... predicates.
The MC Wiki lists their structure in more detail https://minecraft.fandom.com/wiki/Predicate#Predicate_JSON_format.

You can use this site to generate them: https://misode.github.io/predicate/

For item predicates select Match tool and copy the predicate field
For entity predicates select Entity properties and copy the predicate field
For location predicates select Location Check and copy the predicate field
For block predicates select Location Check, define the block and copy the block field inside of predicate

Each entry type has also a multi-variant which allows a quest to have multiple entries that will be resolved when the player accepts the quest. You can e.g. have an multi item entry with multiple tasks defined (coal, iron ingot, gold ingot...) and when the player accepts the quest those tasks will be resolved randomly to a single one (-> iron ingot) allowing more variety in quests.
For NumberProviders used in those entries see https://minecraft.fandom.com/wiki/Loot_table#Number_Providers (though misodes also is able to generate them: Select Value Check copy the value field).
Additionally you can define a custom NumberProvider that will take into account the amount of completed quests to adjust the final amount. Example:

"amount": {
        "type": "simplequests:context_multiplier",
        "multiplier": 1,
        "max": 10
        "value": {
            "type": "minecraft:uniform",
            "min": 10,
            "max": 15
        }
      }

Lastly descriptive predicates are a map of a predicate to its description and will be used in a few multi-types and all follow the same layout of:

{
  "description": "emerald",
  "value": {
    //the predicate
  }
}

Optional descriptive predicates accept both the predicate alone and the above structure.

The following entry-types are supported:

Item Type:

Detects items in players inventory upon submitting the quest

{
  "predicate": <>, //An item predicate
  "description": <>, //Optional. Useful for tag ingredient if the tag has a lot of items to shorten the default description.
  "amount": <>, //Amount of items to hand in.
  "consumeItem": true | false, //optional. default true. If false items will not be consumed when submitting
  "id": "simplequests:item"
}

## **Multi Type**:
```json
{
  "predicates": [], //List of item predicates
  "description": <>, //required
  "amount": <range>, //A NumberProvider that specifies the amount of items to hand in.
  "consumeItem": true | false, //optional. default true. If false items will not be consumed when submitting
  "id": "simplequests:multi_item"
}

Kill Entity Type:

Detects if players kill the matching entity

{
  "predicate": <>, //An entity predicate
  "amount": <>, //Amount of entities to kill
  "description": <>, //Optional
  "id": "simplequests:entity"
}

Multi Type:

{
  "predicates": <>, //A list of optional descriptive entity predicates
  "amount": <>, //A NumberProvider that specifies the amount
  "description": <>, //required
  "id": "simplequests:multi_kill"
}

XP Type:

Player needs to hand in x amount of xp levels

{
  "amount": <>, //Xp levels
  "id": "simplequests:xp"
}

Multi Type:

{
  "amount": <>, //A NumberProvider that specifies the amount
  "description": <>, //required
  "id": "simplequests:multi_xp"
}

Advancement Type:

Player needs to have the advancement when submitting the quest

{
  "advancement": <>, //Id of the advancement
  "reset": true | false //Optional. default false. If true will revoke the advancement for the player upon completion
  "id": "simplequests:advancement"
}

Multi Type:

{
  "advancements": [], //A list of advancement ids
  "description": <>, //required
  "reset": true | false //Optional. default false. If true will revoke the advancement for the player upon completion
  "id": "simplequests:multi_advancements"
}

Position Type:

Player needs to go to the position

{
  "pos": {
    "x": <int>, 
    "y": <int>, 
    "z": <int>, 
  },
  "minDist": <int>, //Min distance to the given position
  "description": <>, //Optional
  "id": "simplequests:position"
}

Multi Type:

{
  "positions": [] //List of possible positions in above format. Also accepts it in descriptive format
  "minDist": <int>, //Min distance to the given position
  "description": <>, //required
  "id": "simplequests:multi_position"
}

Location Type:

More specifig than the position type. Allows for checks of dimensions, structures etc.

{
  "predicate": <>, //A location predicate
  "description": <>, //A description of what the player must do
  "id": "simplequests:location"
}

Multi Type:

{
  "locations": <>, //A list of descriptive location predicates
  "description": <>, //A description of what the player must do
  "id": "simplequests:multi_location"
}

Entity Interaction Type:

Triggered when the player interacts (right click) with the given entity while holding a matching item

{
  "item": <>, //Optional. An item predicate. If not specified matches every item (empty hand too)
  "predicate": <>, //An entity predicate
  "amount": <int>, //Amount of times this has to be done. Doing it on the same entity will not work
  "consume": true | false, //Optional. default false. If true item will be consumed. Note that if an item normally gets consumed already (e.g. name tags) this should be false or else it will consume 2 items
  "description": <>, //A description of what the player must do
  "id": "simplequests:entity_interact"
}

Multi Type:

{
  "itemPredicates": <>, //A list of descriptive item predicates
  "entityPredicates": <>, //A list of descriptive entity predicates
  "amount": <amount>, //A NumberProvider for the amount needed
  "consume": true | false, //Optional. default false. If true item will be consumed. Note that if an item normally gets consumed already (e.g. name tags) this should be false or else it will consume 2 items
  "description": <>, //A description of what the player must do
  "taskDescription": //Description of the actual task with the item predicate, entity description and amount provided as args. So for "%1$s %2$s %3$s" %1$s is the item description, %2$s the entity and %3$s the amount
  "id": "simplequests:entity_interact"
}

Block Interaction Type:

Triggered when the player interacts (right click) with the given block while holding a matching item

{
  "item": <>, //Optional. An item predicate. If not specified matches every item (empty hand too)
  "predicate": <>, //Optional. A block predicate. At least item or predicate has to be specified
  "amount": <int>, //Amount of times this has to be done. Doing it on the same entity will not work
  "use": true | false, //Set to true for using on a block (right click). Set to false for break the block
  "consume": true | false, //Optional. default false. If true item will be consumed. Note that if an item normally gets consumed already (e.g. name tags) this should be false or else it will consume 2 items. Also if you e.g. have it as "mine a block" the item used to mine it will get consumed
  "description": <>, //A description of what the player must do
  "id": "simplequests:block_interact"
}

Multi Type:

{
  "itemPredicates": <>, //Optional. A list of descriptive item predicate
  "blockPredicates": <>, //Optional An entity predicate. At least itemPredicates or blockPredicates has to be not empty
  "amount": <>, //A NumberProvider for the amount needed
  "use": true | false, //Set to true for using on a block (right click). Set to false for break the block
  "consume": true | false, //Optional. default false. If true item will be consumed. Note that if an item normally gets consumed already (e.g. name tags) this should be false or else it will consume 2 items. Also if you e.g. have it as "mine a block" the item used to mine it will get consumed
  "description": <>, //A description of what the player must do
  "taskDescription": <>, //Description of the actual task with args item predicate, block predicate and amount
  "id": "simplequests:multi_block_interaction"
}

Crafting Type:

Triggered when the player crafts an item

{
  "item": <>, //Optional. An item predicate. If not specified matches every item (empty hand too)
  "playerPredicate": <>, //An entity predicate. Note if you specify an entity type that's anything but a player this will always fail
  "amount": <int>, //Amount of items needed to be crafted
  "description": <>, //A description of what the player must do
  "id": "simplequests:crafting"
}

Multi Type:

{
  "itemPredicates": <>, //Optional. An item predicate. If not specified matches every item (empty hand too)
  "entityPredicates": <>, //An entity predicate. Note if you specify an entity type that's anything but a player this will always fail
  "amount": <>, //A NumberProvider for the amount needed
  "description": <>, //A description of what the player must do
  "taskDescription": <>, //Description of the actual task with args item predicate, player predicate and amount
  "id": "simplequests:multi_crafting"
}

Fishing Type:

Triggered when the player fishes an item

{
  "item": <>, //Optional. An item predicate. If not specified matches every item (empty hand too)
  "playerPredicate": <>, //An entity predicate. Note if you specify an entity type that's anything but a player this will always fail
  "amount": <int>, //Amount of items needed to be fished
  "description": <>, //A description of what the player must do
  "id": "simplequests:fishing"
}

Multi Type:

{
  "itemPredicates": <>, //Optional. An item predicate. If not specified matches every item (empty hand too)
  "entityPredicates": <>, //An entity predicate. Note if you specify an entity type that's anything but a player this will always fail
  "amount": <>, //A NumberProvider for the amount needed
  "description": <>, //A description of what the player must do
  "taskDescription": <>, //Description of the actual task with args item predicate, player predicate and amount
  "id": "simplequests:multi_fishing"
}

Example:

{
  "loot_table": "minecraft:chests/jungle_temple",
  "repeat_delay": 36000,
  "task": "Slay mobs",
  "entries": {
    "zombies": {
      "entity": "minecraft:zombie",
      "amount": 12,
      "id": "simplequests:entity"
    },
    "skeleton": {
      "entity": "minecraft:skeleton",
      "amount": 10,
      "id": "simplequests:entity"
    },
    "rottenflesh": {
      "ingredient": {
          "item": "minecraft:rotten_flesh"
      },
      "amount": 8,
      "id": "simplequests:ingredient"
    },
    "xp": {
      "amount": 5,
      "id": "simplequests:xp"
    }
  }
}

Player need to kill 12 zombies, 10 skeletons and provide 8 rotten flesh and 8 xp levels to complete the quest.
As completion reward the player gets the content of a jungle temple chest.
This quest is redoable every 30 min (36000 ticks)