When you reach Anor Londo, you'll find out. Also a quick tip, when you finally ring both Bells, you'll encounter a new enemy called a Mimic. They're disguised as Treasure Chests and are notorious for killing players immediately upon opening them. To tell whether or not it's a Mimic, you either attack any chest you see 1st, or look closely at the chains on the right side of the chest. Regular ones have the chain placed in a circle while Mimics have theirs straightened out.