Official Aviation Definition
Flaps are high-lift devices mounted on the trailing edge of an aircraft wings. When deployed, flaps increase wing camber and surface area, producing more lift at lower airspeeds and allowing steeper descent angles without increasing airspeed.
In Simple Words
Movable panels at the back of the wings that slide out to let the airplane fly slowly and safely during takeoffs and landings.
Real-World Flight Example
The Airbus A320 extended flaps to Configuration Full prior to crossing the runway threshold at 135 knots.