A potion is a magic liquid that produces its effects when imbibed. Drinking a potion is usually a minor action. Administering a potion to an unconscious creature is usually a standard action. Drawing a potion out of your pack is a minor action.
Potions that restore health or even life are useful to adventurers in dire emergencies. These consumables are not as effective as the healing powers of a white mage or a bard, but many adventurers find it useful to carry one or two healing potions with them on their journeys.