The Mirror of Returning is a magical item, which, when activated, will teleport you to a certain spot which it has previously remembered.
A new mirror will always be black and has no knowledge of any destination. You can not teleport by using it yet.
Rightclick with it while pointing to something to set the mirror's destination. The mirror will magically remember your position and it becomes gray. To teleport to the previously set position, leftclick with the mirror once. This will make a loud sound at both source and destination and creates a lot of visible particles, so you will probably catch the attention of others. You can set a new mirror's destination with a rightclick, but the mirror will then forget the previous destination. A mirror does not have that much memory, after all.