For example, take a 10x6. The first number is the diameter (10). The second is the pitch (6). The pitch is a measurement of how far the prop would pull in one revolution in perfect conditions (in inches).

Lower diameter, higher pitch props will fly an airplane faster but offer slower acceleration, like driving a car in 4th gear. A higher diameter, lower pitch prop will fly slower but offer better acceleration and better performance when flying straight up, like driving in 1st gear.

Typically if you increase the diameter you will have to lower the pich or vice versa. This is because you need to keep the load on the engine within a specified range. To find how much a prop will load an engine multiply the diameter by 2, then add the pitch. So a 10x6 prop will have a loading of 26. Let's say you wanted better acceleration in exchange for a slower top speed. You could install an 11x4 to keep the engine within it's designed RPM range (11*2+4=26, same load as the 10x6).

Like everything else with airplanes, it's a compromise. You can't have a bullet fast top speed with excellent acceleration. Find the loadings of the manufacturers recommended props with the formula. Then find other props in the same load range. Try the different props and stick with the one that you think suits your needs best.