"Blown flaps" are an aerodynamic device used on the wings of aircraft to improve low-speed lift and take-off characteristics. The process is sometimes called a boundary layer control system (BLCS).
Blown flaps bleed air from the engines and force it through slots in the wing flaps of the aircraft when the flaps reach certain angles. The bleed air prevents the boundary layer (slow-moving air that accumulates on the airframe surface) on the upper surface of the flap from stagnating, improving lift.
Boundary layer control systems usefully lower the stall speed of an aircraft, making them useful for STOL aircraft (like cargo transports intended for use on short fields) and high-performance fighters with poor low-speed characteristics. Their disadvantage is that they rob the engine of some thrust while in use, which can take-off performance, particularly in "hot and high" conditions.
The use of BLCS began in the late 1950s on aircraft like the A-5 Vigilante and the F-104 Starfighter, and later found use on civilian airliners.
See also STOL.