Variable pitch rotor
A variable pitch rotor can change the angle of its blades while spinning, which changes how much thrust it makes without changing its speed. A fixed pitch rotor has blades set at one angle, so the only way to change its thrust is to spin it faster or slower. Helicopters use variable pitch, while most small drones use fixed pitch props and control thrust through motor speed.
Example: A helicopter's engine runs at nearly constant speed while the pilot climbs or descends by changing the angle of the main rotor blades.