XP items drop from certain mobs. They grant you xp when you deliver them to the npc that is in need of them. Right click the item in inventory and then click on info to see which npc wants that item. Generally xp item names are written in camel case, e. g. Scimitar Skins or Blodfelag Ear etc.
All the collection tasks are listed on the phoenix wiki:
Some of the custom ui's have those list build in. See https://forum.playphoenix.online/viewtopic.php?f=26&t=494
And there are some spreadsheets around.
You can turn in 10 xp items per level and max 10 items of one kind. Each xp item has a level range attached to it in which it can be turned in. XP items are tradeable and thus can be found at the merchants in the housing zones.