Wtf is one? I have been doing some power cleans from the hang, and thinking about expanding my "olympic lifts" and apparently I need one of these.
Actually I meant to say I've been doing power snatches but anyway...
It is simply a platform that absorbs the shock of dropping the weight (bumper plates). Take a look at Coll Col's videos when he is performing OLs. He is on a platform. You can also look at the York Barbell website; they sell them.
Basically a plywood platform which you stand on to execute your Olys so you can ditch the weight without damaging the floor.