Lift forces result from pressure differences between the air flowing under and above the wing. The force is the pressure difference multiplied by the wing area, so a larger area means more lift. Similarly, a larger angle of attack (up to a critical point) increases the pressure difference, also increasing lift. These techniques to boost lift are especially important when the aircraft isn't moving that fast, which is true during takeoffs and landings. At cruising speed, the flaps are pulled in because the aircraft is moving fast enough that the extra wing area and lift aren't necessary.
Work Step by Step
This is discussed on page 274.