For the best flight characteristics. Electric radio control airplanes need propellors and gearboxes selected according to the flight profile desired. If you’ve got a slow flying trainer then you’ll need a large diameter, low-pitched prop mated to a high ratio gearbox. So you might have an 11.5″ x 4 (pitch) prop and a 6:1 gear reduction unit. For a higher speed, higher climbing powered glider, you might select a 11″ x 5″ folding prop with a 5:1 reduction. In the first case, the trainer, you want slower but longer flights, so you don’t need to extract lots of power from your battery-motor combination. The glider however, needs more power, but for shorter times to get up to altitude, when the motor then will be shut down.
Pitch is a relative measure of the amount of twist in the propellor blades. Higher number, more pitch = faster airspeed. If the pitch is higher, then every turn of the propellor moves it further through the air than a lower-pitched prop.
Reduces the number of propellor revolutions relative to the motor so that the propellor can be larger, slower, and more efficient. Think of a big fan, turning slower but moving lots of air. Of course, if you want to go faster, use less reduction, higher propellor speeds all at the cost of shorter, but faster flight times.
Exactly how you make these choices is to try various combinations, but first try what the electric radio control airplanes instructions suggest. If you get really into the project, there are several good software programs available to help you make your choices: Moto-Calc and Electi-Calc allow you to enter detailed information about your plane, and then suggestions will be made as to the best combination of motor, battery, controller, propellor and gearing. Lots of fun on a rainy day!