Pitch is the THEORITICAL distance (in inches) a prop will travel through the air in one revolution. There is always slippage so this figure is never totally accurate.

GIVEN THE SAME RPM:
Lower pitch and larger diameter gives more thrust but less top speed.
Higher pitch and smaller diameter gives more speed but less thrust.

For example, a .40 engine on a pylon racer may use a 9-8 prop. It'll go like stink, but takes a full lap to get up to full speed. An old-timer with the same .40 may use a 11-6. It flys very slowly, but gains top speed quickly.

Generally, planes with more drag need a prop with more pull than speed.

Dr.1