The second number refers to the pitch. That's how far the propeller would move forward with one revolution if it was 100% efficient (no slippage). The first number, or course is the diameter.

So, a 10X6 moves 6" forward in one complete revoultion while the 11x5 only moves 5". Obviously, you would think, the 10x6 would make the plane faster. But, as is often the case in the world, it's not always that easy. Propellers are maybe 35% efficient, the engine won't load up above a certain RPM regardless of the prop and will have a "sweet spot" at which diameter and pitch are optimum. Your trainer has a lot of drag, so a longer prop probably does have the advantage of adding torque at the mid & high RPM ranges where the added speed potential of the 10x6 would be lost in slippage and drag.

To learn all the flies in the ointment read here: [link=http://en.wikipedia.org/wiki/Propeller]propeller efficiency[/link]