It's a grinder that collects trichomes through a mesh screen underneath the grinder (which has holes to allow pot to fall between,)

This gives you a little tray thats filled with kiff/keef/unpressed hash after a while which makes a nice change to herb occasionally (or is good as a booster).

Its a shame the proper metal ones are so expensive, i got a cheaper acrylic model but the aluminium ones have better, sharp teeth.

Jonny