Mhsetup is part of the mh bundle, which provides a series of packages designed to enhance the appearance of documents containing a lot of mathematics. The main backbone is amsmath, so those unfamiliar with this required part of the LaTeX system will probably not find the packages very useful.

The mhsetup package defines various programming tools needed by both empheq and mathtools. In the future, most of these tools will probably be an integral part of LaTeX3. The author is Morten Høgholm. See also amsmath.