A shock is part of your car's suspension that works with the springs.
Without the shock, your vehicle would bounce continuously on its springs. When the shock is working properly, it controls the spring and gives you a smooth ride.
Shocks generally need to be replaced about every 50,000 miles. There are also other signs, like dents in the pistons and leaking fluid, that indicate that your shocks need to be replaced.
Watch the "Driver's Ed" segment above for more information.