The SD is the average 'distance' of the observations in a particular variable from the mean of that variable. Usually, for finding the mean, or average, we divide by n, the number of observations. However, it happens that when the number of observations is less than 30, dividing by n slightly distorts the result when using the normal distribution. The distortion can be corrected by dividing by n-1.
Here is a good worked example which you can alter.