Basically, you'll be rating a poster's sig/avatar that is a post above you. For example..
Dragona: I give this a ?/10, because there's no one above me.
Random Person: To Dragona, your sig and avy... IT SUCKS!
Dragona: *shoots him*, take that random CPU! This is for PC characters ONLY!
Well, you get the gist of it.
Since there's no one above me, go ahead and rate mine. then, rate the next poster's sig/avy, and so on. It's fun.