An item is an object which the player can pick up, keep in their Bag, and use in some manner. They have various uses, including healing, powering up, helping one to catch Pokémon, or to access a new area.
Items are obtained in several different ways. They can be given to you by characters within the game, be bought at a Poké Mart for money, or found throughout the Pokémon world. The latter form of items are contained within item balls, spherical containers resembling a Poké Ball. To obtain the item, move next to it and press Ⓐ while facing it.
Items can also be hidden from view rather than found in an item ball. These items typically aren't visible to you without the use of an item-finding tool. Many times, a seemingly empty path can lead to a hidden item. In Pokémon Sun and Moon, you can call for a Stoutland via Poké Ride, which can help you find hidden items.
Items are separated in the Bag into the following categories:
The general Items pocket contains all objects that are not put in another pocket. Some of item pieces in the Items pocket can be of the following variety:
Held items are items given to a Pokémon to hold onto by its Trainer. Many of these items can be used by the Pokémon holding them. These types of item pieces can also be found in the Items pocket of the Bag when not held:
The Medicine pocket contains various items that can heal various afflictions of a Pokémon. Some of item pieces in the Medicine pocket are:
The TM pocket contains TMs (Technical Machines), items which when used, teach compatible Pokémon a move, providing a wider pool of moves for Pokémon to learn from. TMs are reusable and cannot be discarded.
The Berries pocket contains Berries, items that can be found and cultivated. Many can be used on a Pokémon or given to a Pokémon. Berries have various effects, including HP and status condition restoration, stat enhancement, and even damage negation.
The Key Items pocket contains important Story objects and other key item pieces that generally can only be obtained once in gameplay and cannot be traded between games. Often these are items which you must deliver to a non-player character, but are at other times intended to be kept and either aid the player travelling or the help progress through the storyline.
Key Items rarely have anything to do with your Pokémon and are mostly specific to the game. They can never be discarded, however, certain items will disappear from your Bag during storyline events.
The Z-Crystal pocket contains Z-Crystals. A Z-Crystal is an item required to upgrade moves to Z-Moves. These Z-Crystals can be used to create held items of the same name at will.