The rover body is called the warm electronics box, or "WEB" for short. Like a car body, the rover body is a strong, outer layer that protects the rover's computer and electronics (which are basically the equivalent of the rover's brains and heart). The rover body thus keeps the rover's vital organs protected and temperature-controlled.
The warm electronics box is closed on the top by a piece called the Rover Equipment Deck (RED). The Rover Equipment Deck makes the rover like a convertible car, allowing a place for the rover mast and cameras to sit out in the martian air, taking pictures and clearly observing the martian terrain as it travels.