Feathering a prop means turning the blades sideways, parallel to the direction of travel. This minimizes air resistance. It is typically done on modern gliders, but in theory may be done on any human-controled variable pitch propellor. I bet there are exceptions. Some company has to make electric or mechanical nannies that interfere.
Note that gliders may be planes that are intended to glide and may be propellor driven. They become motorized gliders or self-launching gliders, not planes. The licensing is different.