In Battle edit
When in battle, a Pokémon flies up, causing the opponent's Pokémon's attack to miss.
Then on your next turn the Pokémon flies down and attacks the opponent's Pokémon, causing serious damage.
Overworld Use edit
Prior to generation 7, this move enables the player to quickly fly to any town visited when out of battle. In these generations Fly was often taught by an HM, letting it be reused multiple times.