Its clay
Its used to remove microparticles of dirt of the paintwork, but you have to be careful with it as it is fairly aggressive on the protective coating on your paint, so better only use it before applying a new layer of wax to the car, and not after every wash
twice a year it's plenty for claybarring.
It's not "aggressive" as you put it, but there's no point in using it after waxing, as it'll strip the wax as well as the dirts
It's one of the most effective things I've ever used. It's amazing how much crap it takes away from the paintwork, which you can't even see!
The proofs are 2:
1- after finishing washing the car, pass a hand onto a panel. You'll feel lots of invisible crap stuck to it. Do it again after claybarring....
2- look at the claybar surface after an application before kneading it, and see how much crap it has taken off your paint