The first number is the length of the prop and the second number is the pitch.

Roughly speaking and without getting all technical.... A prop with a "5"pitch pulls the prop forward 5 inches with every turn and a prop with a "6"pitch pulls the prop forward 6 inches every turn etc, etc.

So a higher pitched prop will fly the plane faster and a lower pitch prop will fly slower, just make sure you stay with a prop in the correct size range for the engine.