Flight explained
Why aircraft need flaps
How high-lift devices help aircraft take off and land at lower speeds.
The short answer
The flaps extend from the rear edge of the wing and change its curvature, and in some modes increase the effective area. The wing creates more lift at the lower speed required for takeoff and landing.
How flaps change lift and speed
Increasing the lifting force has a price - the resistance is growing. On takeoff, the flaps are released moderately, and on landing, a greater deviation is used, which additionally helps to descend without acceleration.
After breaking off and dialing a safe speed, the crew removes the mechanization in stages. Sharp cleaning would reduce lifting power, so each step is associated with a given speed.