Pitch refers to the angle of the blades on a helicopter or propeller compared to a fixed datum. The angle of attack of the blade is determined by the pitch and the speed air is flowing through the propellor/rotor.
The angle of attack of an airfoil determines the lift and drag produced. Thus the pitch of a propellor/rotor blade and the airspeed of the aircraft affects the torque required to drive it, as well as the thrust produced.
When applied to propellers, pitch indicates the distance a propeller would move through a static medium per revolution.