Of course the first standard deviation is a measure that shows the level of variation among a set of values, and is of course derived by taking the sqrt of mean squared differences of the values to their mean.
But what if you needed to know the level of variation OF the variation of the set of values. This would be the second standard deviation, and would be derived by taking the sqrt of mean squared differences of the residuals to their standard deviation. And in the same way: the third, fourth, and nth standard deviation.