What does 925 mean on jewelry?

I got a little jewelry necklace piece without the chain and i would say its fairly heavy for its size (slightly bigger than a dime *its also square-ish with an 'M' design in the middle*). It looks kinda tarnished all over and on the back of the piece there is the number 925. I don't see any thing else. But what does that mean? Is it worth anything?