What Chuck says is correct, but just to addd...
ROTOR RPM is the constant, however the more collective pitch you pull the more ENGINE RPM is required to overcome the drag of the increased pitch. This is mostly in the hover, as with forward airspeed (or reverse & sideways if fast enough
) you get "transational lift" which alters air flow angles (too tired to go into more detail, yawn) and increases lift, therefore less engine power required........