ok here is a small explanation about the pitch.

excuse my poor english.

for a specific prop size, as less is the pitch as hig is the rpm (and vice versa).
to explain what is the pitch try to imagine your propeler turning in something like mayonaise. a six inches pitch prop will enter the mayonaise 6 inches in one revolution

for a specific pitch, increase the prop diameter will reduce the max rpm (and vice versa)

the basic effect of pitch on flight performance is as follow. the pitch can be compare to the "gear" of a car. a small pitch will give you a hig rpm, the plane top speed will be slightly lower, but the take off run will be shorter and the climb performances better.
A hig pitch prop will give you a faster plane, but take of run will be longer and climb performances reduced.

try to always select your prop to stay in the engine manufactuer RPM recomandation

that's the basics