Medians as a concept are simple enough. If you have a large number of values, like a range of statistical values, you want to pick the middle one. The median, as opposed to the average is useful for a number of reasons, one of them that you can reduce the effect of so-called outlier values.
The fact that SQL Server doesn’t have a fast, built-in median function surprises me, to be honest. The best alternative I’ve found was a CLR function in SQL#.