The Mirror of Erised is a magical artifact which appears in Harry Potter and the Philosopher's Stone, the first of the four (so far) books. The inscription on it reads:

Erised stra ehru oyt ubeca fru oyt onwoh si (I think).

My girlfriend thought for a while that that was Latin, and asked me to translate that for her. If you still haven't seen it yet, it's backwards.

i.e. I show not your face but your heart's desire.

Spoiler: Harry uses this mirror to locate the Philosopher's Stone during his showdown with Voldemort at the end of Book 1.