Rolling a fender is a process of permanently altering the shape of the metal to make it more pliable. This is done by heating the metal until it's soft enough to be worked, then using a hammer and dolly to slowly beat it into submission. The advantage of rolling a fender is that it can be used to improve the looks or function of a car without having to replace the entire panel. For example, if you have a car with wide tires and want to avoid tire rub, you can roll the fenders so they flare out slightly and give you more clearance.
However, there are also some downsides to rolling fenders. One is that it can decrease the value of your car. If you're planning on selling or trading in your vehicle, rolled fenders will likely be seen as a negative by potential buyers. Another downside is that rolling fenders isn't always reversible. Once you've heated up and hammered out the metal, there's no going back (short of replacing the entire panel). So if you're not sure about how you want your car to look, it's best to leave the fenders alone.