Propeller pitch is the theoretical forward movement of a propeller for one revolution, assuming that there is no prop slip. For example, a 20 pitch propeller will theoretically move 20 inches for every revolution. Propeller slip occurs with every propeller, but the amount of slip varies depending on propeller design. More aggressively and efficiently designed propellers will slip less.
When selecting a propeller pitch for your boat, it is important that the propeller runs at the upper end of your engines wide-open-throttle RPM range. If you want your RPM's to increase, go down in pitch. To decrease RPM's, go up in pitch. As a general guide for inboards, for every 1" of pitch, RPM's will change approximately 100 RPM. The ratio for changing pitch will vary since changing 1" on a 30 pitch will have less impact than a 19 pitch. We recommend you start off with getting the manufactures specs for the boat and have the necessary load information when determining if you need to adjust the pitch.
For water sports or extra people on board, you should generally drop 2" of pitch to help compensate for the added weight and drag on your boat. It makes a noticeable difference in your boat's hole shot, fuel efficiency, RPM's, and overall performance. You should ALWAYS carry a spare propeller on board, and if you're into water sports or occasionally load the boat with extra people, a spare prop with a lesser pitch is a good idea. When thinking a propeller pitch, compare it to a gear on car - lower gear, higher RPM's.
Pitch is the 2nd two digits that are listed in a propeller item description (18 x 19, 19 is the pitch, 18 is the diameter)