The L stands for Lovibond and it is a unit of color. The higher the number the darker the malt. So if you use similar amounts of 10L and 60L crystal malts in two beers, the one with 60L will be darker.
Actually one other thing - aside from the color, does it change the flavor at all?
If a brew calls for 60L, but I only have 10L, will that change the flavor? Obviously it will change the color and possibly put it out of style, but anything else besides that?