The Mirror Shield is Pit's Down Special Move that makes him take out a shield that reflects projectiles and prevents any damage done in front of him, as long as it hits the shield, even explosives and some Final Smashes. The reflected projectile does more damage than it would originally, and some will even be accelerated, similar to how Wolf's Reflector works. His Angel Ring is a faster reflector against projectile attacks. However, the shield is very good protection against an attack that has a startup lag, such as Captain Falcon's Falcon Punch or most forward smash attacks. Unlike other Reflectors, Pit actually says "Nice Try!" whenever he reflects a projectile.
It may also turn around enemies, similar to Mario's Cape, if the shield is attacked directly during initiation.
Pit will put his shield away after about 15 seconds if the button is held.
Another interesting thing is that you can angle up the shield a tiny bit. This usually has no effect on the reflection, but interestingly, you can actually block Samus's Zero Laser Final Smash by angling the shield up at the correct distance. This blocks all hits except for the final one.
Pit's shield can be use to to block Tabuu's Dragon Laser. Pit won't take damage but he will be pushed back.
The Mirror Shield is one of the three sacred treasures Pit needed to defeat Medusa. It is based on Greek mythology, where a mirrored shield is used to defeat Medusa by looking at her reflection in it rather than her.