The propeller is the final link in a boat's power train. Its job is to take the torque created by a marine engine and convert this torque to forward movement.  A propeller is designed to control the engine's RPM and hold it at wide open throttle to the recommended RPM range specified by the manufacturer of a motor.

The power curve is the RPM range where the motor is developing its maximum horsepower and efficiently. If the motor revs beyond this point, it is over revving, and if the motor is not able to reach this point it is loaded down, which could have damaging effects in the internal components of the engine, as well as allow the engine to use fuel excessively.  Either way, this causes poor performance and unnecessary wear and strain of the drive train. 

Propeller pitch is speed, however there are also several other factors which allow a boat to perform to it's potential.  Such as, pitch progression, rake, and cupping.   Also the gear ratio of the lower unit is also extremely important to a boats potential for speed.