"Erised stra ehru oyt ube cafru oyt on wohsi"
("I show not your face but your heart's desire")
-- Inscription on the Mirror of Erised (PS12)
The unused 4th floor classroom contained stacks of chairs and desks and an upside-down wastebasket. The room was the storage place in 1991 of the miraculous Mirror of Erised propped against a wall (PS12).
Harry was able to see his parents and a whole host of dead family members in the Mirror of Erised (PS12). When he brought Ron to the room, however, the other boy saw different things, including himself as Head Boy and Quidditch Captain holding the House Cup.
Dumbledore found Harry in the room and explained that the Mirror was not exactly “true” and only showed “the deepest, most desperate desire of our hearts.” He also said the Mirror was going to be moved to a safer place.