A controllable pitch propeller (or CPP) is a form of propeller wherein an articulated hub can be used to control the pitch of the blades. The blades can be feathered, or reversed quickly, and without slowing or stopping the prime mover.
CPP's ability to be quickly varied aides maneuverability. By changing pitch, the vessels propeller can be more efficiently matched to multiple modes of operation.
CPP's introduce cost & complexity into the drivetrain. The larger hub required for the control mechanisms reduces propeller blade area, lowering efficiencies at a single given speed/load design point.