To ride or stunt on 50cc pit bikes or dirt bikes, usually in a group of two or more. Also includes 50cc go-karts, lawn mowers, weed wackers, etc. Mobbin implies that the riders are riding derty.
E.g. you roll up to give your ex back some shirts he's been bitching about with your new boo thang, and when he wants to talk you say nah bro and roll off with new bae in front of his ass