The Moon Bell (馬蹄鈴, also called "Bell of the Moon" or "Bell of Second Chance") is the magical bell possessed by Kaho Mizuki; passed down to her by Clow Reed, when they met at a university in England.
The bell's use is to give Sakura Kinomoto a second chance during the Final Judgement. The Bell vanished after being used during.
It was, also, used to defeat the Maze card from the inside by breaking it's walls; allowing Sakura, Tomoyo, Syaoran, Meiling to escape and seal the card. It was first seen in episode 26.