What is autofeather for?
Autofeather is an automated feathering feature, common in a lot of turboprops, which automatically feathers the propeller on an engine if it loses power without having to either pull the power or propeller lever to feather the prop.  

It reduces crew workload in the event of an engine failure and, as a feathered propeller greatly reduces parasite drag on the side of the dead engine, reduces the asymmetrical thrust yaw moment about the vertical axis of the airplane.  

It is particularly useful during takeoff and landing operations where airspeed may be very close to minimum controllable airspeed on one engine.