A bracelet (or sometimes, an anklet) upon which can be attached numerous small trinkets, or charms. Nowadays, they are most often worn by pre-teen girls, though they used to be a fairly common fashion accessory.
Sometimes, the charms are considered to be lucky, but usually, they are just for decoration or personal expression. Charms can include everything from cheap Cracker Jack toys to fancy (and fairly pricey) plastic or metal charms in a variety of shapes.
Charm bracelets are often young girls' first piece of jewelry, and they allow them to jangle loudly as they walk down the hall at school, which is also considered a plus.