Pitch is the distance the prop would move foward in one revolution. So if you have a 21 pitch prop one rev. would push the prop 21 inches foward minus some slight slip. Changing the prop pitch is like changing the gear ratio or tire size on a car. A smaller pitch prop would traditionally be used when a better/faster holeshot is desired but should be used with caution due to the potential for over revving the motor and the larger the pitch the higher top end speeds. There as far as I know is no real science to finding the right combo. Try and find the prop that will give you a good hole shot and good topend speed without over revving go find a 21 and give it a try.