Flaps are high lift devices that are normally fitted to the trailing edge of the wing to increase the wing area, and thus lift, for any given airspeed. These are used for landing on almost all aircraft but are also used for take-off on most airliners.
You may have heard that the Boeing 747 was the first aircraft to have the triple slotted flap system, but what does this actually mean? Triple slotted flaps extend downward and rearward away from the wing in three sections. The trailing edge of one section forms a duct with the leading edge of the section behind it to force air down over the top of the flap.
Triple slotted flaps prevent the flow of air from separating from the surface of the flap when they are fully extended. When not in use, the flaps are flush with the wing.