The pitch refers to the angle of the rotor blade. Positive pitch would mean the leading edge is a bit higher than the trailing allowing the blade to take a (bite) out of the air. Fixed pitch means that this angle is preset when the blades are made and so if you want to go higher the blades have to turn faster. With collective pitch the pitch is controlled by the pilot so if you wanted to go higher just give it more pitch and the blades will take a bigger bite out of the air without increasing the speed of your motor.