Side sills are structural components on a vehicle that provide support and strength to the sides of the car. They are also known as rocker panels, and they run along the bottom edges of both front and rear doors, as well as the entire length of the vehicle's side. The purpose of side sills is to protect the body from road debris, provide a smooth transition between the body and the wheels, and add structural integrity to the car. They are typically made out of metal or plastic materials that can withstand wear and tear over time.