Faint is the condition when a Pokémon's HP depletes completely (drops to 0). That Pokémon cannot battle anymore (however, it can still use in-field moves that it knows, such as Fly) unless a Revive, Max Revive, or Revival Herb is used, or it is healed by a Pokémon Center. Sometimes during an adventure in gameplay, if there's a trainer fighting alongside you, they will heal your Pokémon after every battle or before a tough battle against a boss. Nurses and doctors in Black, White, Black 2, and White 2 heal your Pokémon every time you talk to them after defeating them in battle the first time you talk to them, so you don't have to travel all the way back to a Pokémon Center. Most of the time Pokémon will grow to like you more if you don't just rely on Pokémon Centers for healing your Pokémon, but using Revives and Potions to keep them from fainting as long as you don't run out of Pokémon. When all of your Pokémon have fainted, you will lose money and get sent straight to a Pokémon Center nearby. However, if you have many Pokémon fainted and you don't have any Revives, Max Revives, or Revival Herbs, then a Sacred Ash can be used to heal all fainted Pokémon at once.