Have you ever noticed large panels on the upper surface of the wing just forward of the flaps that open partially during the approach, and then open all-the-way once the airplane has touched down on landing? These panels are called spoilers, and unlike lift-enhancing devices such as trailing-edge flaps or leading-edge slats, they are used to actually decrease lift. They also deploy asymmetrically to augment the ailerons on each wing for banking the aircraft, but are deployed together during the approach and landing.
Spoilers serve several very important functions. In flight, they effectively increase the aircraft’s rate-of-descent without causing an accompanying increase in airspeed. Once the main landing gear is firmly on the runway, the pilot deploys the spoilers to their fully extended position, creating the maximum amount of drag to slow the aircraft from its touchdown speed of approximately 120-to-140 mph to a more proper braking speed of 60-80 mph. Once the aircraft has decelerated to taxi speed, spoilers are stowed.