Define standard deviation. Define mathematical averages.

Theaverage is simply the average of a set of numbers. The standard deviation is a measure of how spread out the numbers are with respect to the average. ~64% of the numbers will be within 1 standard deviation of the average.

The mean (or average) is just an average over your sample of numbers, such as (n1+n2+n3)/3. The average does not have to be a number in your sample set, but does have to be in between your sample.