The first number is the length, and the second number is the pitch.
For an 8x6 prop, it is 8 inches long, and in a theoretical "perfect world", it would move forward 6 inches for every revolution.

The pitch and length work together to create the load for the engine, and each engine runs best within a given load range. For example, if you put a 12x6 on a .20 size engine, the engine would run poorly and overheat.
Putting a 9x6 on a .61 engine would not provide enough load, and would cause the engine to over speed, possibly damaging it. This becomes even more critical on 4-stroke engines.

So you have an engine that is happy within a given load range... let's go back to the .20, which is very happy with a 9x6 prop. What other props could you use, and why?

A 9x6 prop has about the same load factor as an 8x10, or a 7x16(!), and also a 10x4, or an 11x2.5

So what's wrong with the 9x6?
It depends on the application.

The 10x4 will provide about 50% more static thrust than the 9x6. This means that the plane will have better ability to pull vertical uplines, perform 3D "hovering" maneuvers, or simply "horse" your way out of a stall.

Well... if the 10x4 provides so much more static thrust, then why wouldn't you ALWAYS want it, or even go to the 11x2.5?

That thrust comes at a tradeoff of speed.
At 13,000 rpm, the 9x6 will be driving a reasonably slick airplane at close to 75mph. The 10x4, at the same 13,000, will be closer to 50mph.
Hmm... 50% increase in speed or thrust... take your pick.

Okay... what if you want to go faster?

Smaller prop... higher pitch.

The 7x16 will have the plane going close to 200mph... but offers LESS THAN HALF of the static thrust of the 9x6.
This prop/engine combination would have to be used on a VERY slick airplane with little drag... retracts and a low aspect ratio wing would be required. Even then, it would take a lot of runway to get off the ground, and take a lot of time to build up speed.
This is the extreme... a "speed prop" would actually be an 8x8 to 8x10 and offer just over 100mph on the right airplane.

Airplanes with a lot of inherent drag, such as biplanes, or trainers with flat-bottomed wings, will generally fly best with a prop that is larger in size and lower in pitch, but you will almost always want to stay near the middle of the recommended prop range for your engine.