Sounds like what it isit's a small lump of clay that you rub over lubricated paint and it picks up a lot of the tar and other debris that's lodged into the clear coat of your paint. When I've clayed friends (neglected) cars after doing it they always look a million times better!
So i just get the bar and rub it all over the car? Do you need to wipe any residue or anything off afterwards?