From the Super Mario Wiki
The Mirror Shield is one of the Three Sacred Treasures in the Kid Icarus series that appears in Super Smash Bros. Brawl as Pit's down special move. When used, Pit crouches and holds a shield up in front of himself that reflects oncoming projectiles, also causing them to deal more damage. It can also protect against physical attacks, though only when the attacker is facing Pit. The player can keep the shield out longer by holding down the special button, and can also angle the shield upward with the control stick.
This move is replaced with the Guardian Orbitars in Super Smash Bros. for Nintendo 3DS / Wii U. The Mirror Shield still appears in Pit's Final Smash, Three Sacred Treasures, though it is only aesthetic.