The term "rocker panel" generally refers to an exterior body part found on most passenger automobiles. It is a panel that can be made out of metal or fiberglass and is located below the side doors. It typically runs the length of the exterior part of the vehicle that is between the front and back tires to conceal the outer frame rail.
A rocker panel is designed to take the brunt of tar, oil, rocks, pebbles and other debris that comes flying out from under the front tires. This helps protect the sides of the vehicle from damage, such as chips in the paint, which can contribute to rust. Typically, it is cheaper to replace just this panel than it is to replace a damaged door panel. There are special coatings that can be applied to these panels that make them more resistant to chipping, rust, and other damage, as well. These coatings might be offered directly from the manufacturer or from a local auto body shop.
Not only can this part provide protection, it can also add to the aesthetic value of a vehicle. Some auto manufacturers offer special panels covered with chrome or painted in a color that is in contrast to the rest of the vehicle. Also, there are many automotive stores that carry decals that are designed specifically for this area. Replacing a vehicle's standard rocker panel with a decorative one is a somewhat easy and fairly inexpensive way to change or add to the looks of a vehicle.