it's just a windshield wiper attached to the top of your imac that wipes the dust away. you can set it to slow (once/day), medium (once an hour), or fast (once a minute). it sprays windex onto the glass and then wipes it dry.
pros: it's faster than spraying windex and wiping it with a towel.
cons: since it makes a curved motion, it cannot reach the far corners.