A Potion is a type of medicine that can be used to heal up to 20 HP on one Pokémon. It can be used by sending out a Pokémon and right-clicking the Pokémon with a Potion in hand. It can be crafted, and can also be obtained from shopkeepers and as a tier 1 special drop.
Three Potions can be used to craft a Super Potion. This heals fewer HP in total, but can be used in one battle turn instead of three.