A "cage" of metal tubing that is put inside a car shell. It reinforces the car so it doesn't crumple if the car rolls over. (You know, how the roof of the car caves in crushing the driver?) Normally, they're made with a strong metal, use triangles in their geometry (due to the fact that triangles are harder to colapse that squares), and joints are weilded solid. Almost all types of professional racing require them on the race cars.