Potions heal Pokémon, restoring HP lost during battle. Potions come in various strengths and restore different amounts of HP. However, if a Pokémon loses all its HP, it will faint and can't be healed with Potions.
Revives are used to revive Pokémon that have fainted. Max Revives fully restore a fainted Pokémon's HP, while standard Revives only bring back a portion of total HP.

To heal or revive a Pokémon:
  1. From the Map View, tap the Main Menu
  2. Tap Items
  3. Select a Potion or Revive from your Bag of items, then select the Pokémon to be healed or revived. You can also tap Heal All or Revive All to apply the item to any Pokémon needing to be healed or revived.