The Held Item is an item that a Pokémon can carry.
Each Held comes with a feature called Tier, which is nothing more than the level, it means, Held items with a higher tier will be better and rarer.
Your Pokémon can only carry two Held Item (one with prefix 'X' and other with prefix 'Y'). It is possible to replace the Held Item in your Pokémon, but you will lose the one that is replaced.
List of Held Item X:
List of Held Item Y: