Heat shields are used in a car's exhaust system to protect various nearby items from the high heat the system generates. Shields are usually found on the top and bottom of the exhaust system.
The top heat shields are used to keep the floor of the car cool. The bottom shields are used to prevent combustible materials that might be underneath the car, such as grass, from catching on fire. Heat shields are often made of sheet metal and sometimes rust and break free due to constant exposure to the elements. Modern heat shields are often made of aluminum and sprayed with a protective coating to prevent rust.