I'll try. Basically, the standard deviation tells you where the majority of your salaries are - namely, more than 2/3 are at most std dev away from your average salary, and more than 9/10 are at most 2*std dev away from it. It is additional information beyond the average salary to tell you if there are one or two guys having a large influence on the salary or not.
I could also give you the formula, but that one is quite complicated if you haven't had probabiliy theory yet (which would be unlikely in jr high math).