Why do planes have flaps on the wings?

Airplane wings are designed to be most efficient when at high altitudes, traveling fast, while providing as little drag as possible. Unfortunately this design inst the best for takeoff and landing. Flaps are used to change the shape of the wing to accommodate takeoff and landing requirements.

Most planes have flaps on the front of the wings (leading edge flaps) and at the back of the wings. During takeoff flaps increase lift so a plane can take off with limited runway length and lower speeds. As the plane gains speed the flaps are retracted. On approach and landing you want to fly as slow as possible so the flaps are deployed in increments till landing speed is achieved. Landing slower means less runway needed to stop.