Here, the sigma means sum each data point from i=1 to i=N. This sum is then divided by the number of measurements (the number of data points, or 'N')

For example, for the data set (1.00, 5.00, 7.00, 3.00, 5.00, 7.00, 8.00), N = 7.

Using the equation, = (1.00 + 5.00 + 7.00 + 3.00 + 5.00 + 7.00 + 8.00) / 7 = 5.14