Fly is Charizard's Up Special Move. Performing the move makes Charizard rise up while surrounded in flames, setting anyone that makes contact with it ablaze. The ending hitbox has somewhat forceful knockback, which can make this move good for juggling. Frames 4-13 of this attack provide Launch resistance. This can be most easily shown by performing this move next to a smart bomb; Charizard will take all damage he would normally but will not be knocked back.
Origins
This attack has its origin in all the Pokémon RPG games for handhelds. As an HM, it can allow the user to travel from town to town very easily provided that a Pokémon knows it, the badge that allows its use outside of battle, and you have been to the city or town you are flying to. Due to this, many have Flying Pokemon on hand to cross long distances in seconds. In battle, it allows the user to avoid almost any attack on the first turn and then deals damage to a single opponent on the second. Interestingly, the move in Brawl seems to be a real-time interpretation of its in-game stats in the Pokemon games, as it leaves the user vulnerable if it misses, but its modest power is decent enough for moderate risk of use.