The pitch of the propeller refers to the distance (forwards or backwards) the propeller moves in describing one complete rotation - this can simply be likened to a screw being screwed into a piece of wood! As water is not a solid substance the propeller will move some 70 to 90% of this distance. The remaining 10 to 30% difference is known as “slip”.
Changing the pitch is very simple; it is done by changing the setting of the gear inside the propeller (usually upon a haul out). The ability to adjust the pitch will help achieve the maximum efficiency from the engine without having to purchase a new propeller.