To wear your trousers in a silly looking way around the lower half of the upper legs, inspired from the 'prison bitch' look in American prisons, where orange overalls would be worn looser to apparently allow easier access to the anus. The style is usually worn by people who think they are 'gangster' but are actually not.
Low rider: Thoughts I look bad in low ridin' ma trousers.
Smart person X: Y, look at that chav thinking he is bad wearing his trousers like that.
Smart person Y: Yeah, what an idiot.