The IPlayer interface allows you to view certain information on a specific player and interact with said one. Mostly used in Event Handlers and Recipe Functions.

It might be required for you to import the package if you encounter any issues (like casting an Array), so better be safe than sorry and add the import.
import crafttweaker.player.IPlayer;

IPlayer extends IEntityLivingBase. That means all functions available to IEntityLivingBase Objects also are available to IPlayer Objects.
IPlayer also extends IUser. That means all functions available to IUser Objects also are available to IPlayer Objects.

Zengetters are for retrieving information. Usually either assigned to a variable or used in a method/function.

Zengetter	What does it do	Return Type	Usage
Zengetter uuid	What does it do devuelve el UUID del jugador	Return Type string	Usage player.uid
Zengetter name	What does it do returns the player's name	Return Type string	Usage player.name
Zengetter data	What does it do returns the player's data	Return Type IData	Usage player.data
Zengetter xp	What does it do returns the player's experience level. Can also be used to set a player's experience level	Return Type int	Usage player.xp
Zengetter hotbarSize	What does it do returns the player's hotbar size	Return Type int	Usage player.hotbarSize
Zengetter inventorySize	What does it do returns the player's inventory size	Return Type int	Usage player.inventorySize
Zengetter currentItem	What does it do returns the item the player is currently holding	Return Type IItemStack	Usage player.currentItem
Zengetter creative	What does it do returns if the player is currently in creative mode (a.k.a gamemode 1)	Return Type bool	Usage player.creative
Zengetter adventure	What does it do returns if the player is currently in adventure mode (a.k.a gamemode 2)	Return Type bool	Usage player.adventure
Zengetter x	What does it do returns the player's current X position in the world	Return Type double	Usage player.x
Zengetter y	What does it do returns the player's current y position in the world	Return Type double	Usage player.y
Zengetter z	What does it do returns the player's current z position in the world	Return Type double	Usage player.z
Zengetter position	What does it do returns the player's current position. Can also be used to set a player's position	Return Type Position3f	Usage player.position
Zengetter foodStats	What does it do returns the player's foodstats.	Return Type IFoodStats	Usage player.foodStats
Zengetter bedLocation	What does it do returns the location of the player's bed.	Return Type IBlockPos	Usage player.bedLocation
Zengetter fishHook	What does it do returns the player's fishingrod entity.	Return Type [IEntityFishHook ](/Vanilla/Entities/IEntityFishHook /)	Usage player.fishHook
Zengetter foodStats	What does it do returns the player's foodstats.	Return Type IFoodStats	Usage player.foodStats

ZenMethods son para hacer cosas con otras cosas, en este caso con un jugador.

ZenMethod	Parameter Type(s)	What does it do	Ejemplo
ZenMethod removeXP(XPtoRemove)	Parameter Type(s) int	What does it do Removes the given experience levels from the player.	Ejemplo player.removeXP(1)
ZenMethod update(IData)	Parameter Type(s) IData	What does it do Updates the playerdata to the provided IData.	Ejemplo ​
ZenMethod sendChat(Message)	Parameter Type(s) string OR IChatMessage	What does it do Sends the player a Chat Message.	Ejemplo player.sendChat("Hello my old friend")
ZenMethod sendStatusMessage(message, hotbar)	Parameter Type(s) string OR IFormattedText	What does it do Sends the player a status message, if the hotbar argument is true, the message will be displayed on player's hotbar, the default value is true	Ejemplo player.sendStatusMessage("hello, world")
ZenMethod sendRichTextStatusMessage(message, hotbar)	Parameter Type(s) ITextComponent	What does it do same as sendStatusMessage, but the message is an ITextComponent	Ejemplo ​
ZenMethod getHotbarStack(index)	Parameter Type(s) int	What does it do Returns the item at the given index within the player's hotbar.	Ejemplo player.getHotbarStack(3)
ZenMethod getInventoryStack(index)	Parameter Type(s) int	What does it do Returns the item at the given index within the player's inventory.	Ejemplo player.getInventoryStack(3)
ZenMethod give(item)	Parameter Type(s) IItemStack	What does it do Give the player the provided item. Item is an IItemStack.	Ejemplo player.give(<minecraft:gold_ingot>)
ZenMethod teleport(position)	Parameter Type(s) Position3f	What does it do Teleports the player to the provided position in the same dimension	Ejemplo player.teleport(position)
ZenMethod executeCommand(raw)	Parameter Type(s) string	What does it do Executes the command as the player	Ejemplo player.executeCommand("kill")
ZenMethod dropItem(dropAll)	Parameter Type(s) bool	What does it do Drops the current item (or the entire stack) that the player is holding.	Ejemplo player.dropItem(false)
ZenMethod dropItem(itemToDrop)	Parameter Type(s) IItemStack	What does it do Drops the provided item at the player's position.	Ejemplo player.dropItem(<minecraft:dirt>)
ZenMethod getCooldown(item)	Parameter Type(s) IItemStack	What does it do Gets current cooldown of specific item for the player	Ejemplo player.getCooldown(<minecraft:ender_pearl>)
ZenMethod setCooldown(item, time)	Parameter Type(s) IItemStack, int	What does it do Sets cooldown time of specific item for the player	Ejemplo player.setCooldown(<minecraft:ender_pearl>, 3)