Flaps are hinged surfaces that are usually located on the trailing edge of the wings on fixed-wing aircraft and are used for various purposes. Flaps that are located on the leading edge of the wings are known as slats and/or Krueger flaps.
Flaps increase the camber of the wing airfoil, thus increasing the overall lift on the airfoil. This extra lift allows the aircraft to fly at slower speeds while still generating enough lift to remain airborne at the reduced speed. In addition to changing the camber of the wing, some trailing edge flap systems also increase the platform area of the wing. By increasing the overall area of the wing, the aircraft is able to generate a larger amount of lift with a slower speed, further reducing stalling speed.
Due to the lifting characteristics of flaps, pilots often use them during takeoff and landing to reduce the stalling speed of aircraft. Extending the flaps also increases drag on the wing which can be useful in slowing the aircraft for approaches. Flaps increase the drag coefficient of an aircraft because of higher induced drag and are usually fully extended for landing which allows the aircraft to land in a shorter distance.