Feathering allows the pilot to adjust the angle of the prop blades. DIfferent settings are used for takeoff, climb and cruise speed. In multi-engine planes when on engine is lost, it can be feathered to stop the prop turning (there may be some kind of brake or clutch mech on twins/multis to stop the prop spin completely, Im not sure), which makes the plane easier to fly with one engine out.
strk