Potions heal Pokémon, restoring HP lost during battle. Potions come in various strengths are 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:
From the Map View, tap the Main Menu
Select a Potion or Revive from your Bag of items, then select the Pokémon to be healed or revived.