what is it to roll fender?
why is this done?

 

There is a flange on the inside of the fender which can rub on the tires when the chassis drops a certain amount. Usually after a dip in the road.

Rolling the fenders begins with bending that flange up against the inside wall of the fender to gain more clearance. If that doesn't work, you can actually go further by now bending the actual fender up higher but that is not recommended and can be easily avoided.

It is done by many with a baseball bat pinched between the top of the tire and the fender and the car is slowly rocked back and forth while the fender gets "curled" up.

Again, it is very avoidable with the correct size and offset of the wheel/tire setup.