Wings on a racing car are upside down compared to an airplane wing. Instead of lifting the car, they press the car harder onto the track, providing increased traction for braking, acceleration and cornering. Downforce is also provided by the ground-effects tunnels underneath the car, creating a vacuum that sucks the car to the track. A modern Champ Car provides so much downforce that it could actually stick to the ceiling at just over 100 miles per hour. Increased downforce also results in increased drag, which slows a car down, so it's a tradeoff.
The air pressure flowing over the car creates a downward force that pushes a car onto the track, causing it to stick to the racing surface. It keeps cars from losing traction at high speeds, especially going through the turns.