For example, let's say Company XYZ stock has the following prices:

The average of these prices is $21.33. To calculate the variance, we see how "far away" each day's stock price is from $21, like this:

Notice that some of the differences are negative. Because we're going to calculate the average difference, the negative numbers create a mathematical problem (they'll offset the positive numbers and screw up the calculation). To avoid this, we square each difference so that each difference is positive, like this:

The last step is simply calculating the average of those squared differences, which is $9.42, and then taking the square root of that number to get the amount by which Company XYZ stock tends to vary from its average price.

The square root is $3.07, meaning that when Company XYZ deviates from that $21 average, it tends to do so by about $3.07.