Basically, to describe your example, the 2-9 is the range of magnification that would be available on a variable scope with a 2-9 range. Basically, you can adjust your magnification from 2x to 9x magnification. A fixed power scope, ie 10x42, has a fixed magnification of 10x. As for the "50" part, that is the size of the objective lense in mm. Basically, the size of the objective lense will determine how much light enters the scope. Theoretically, more light, brighter and clearer picture. However, the quality of the glass will be the determining factor on how clear and bright your picture really is.
I am sure there is some much more technical info to explain all of this, but that is the basics for now.
