Most flaps used in RC simply increase the angle of attack of the wing. This basically increases the incidence of the wing relative to the fuselage and allows the plane to produce a similar amount of lift at a reduced speed while the plane keeps the same attitude in the air.

This higher angle of attack allows more lift to be created, which also creates more drag. This also gets you closer to your critical angle of attack, which is why you will never see flaps only on the outboard end of a wing. If you exceed your critical angle of attack, your wing stalls, and if the far end of your wing stalls before the rest of the wing, that wing will drop and you will go into a spin, which can be very destructive if you are not prepared for it.