It is the AS3 version of PureMVC packaged as a HumboldtJS compatible library.
Usually you will only want the bin/PureMVC.swc and drop that into your existing HumboldtJS project. You can safely ignore the rest of this repository. The rest is only needed if you want to fix bugs or want to upgrade the PureMVC version or add a plugin.
Unless you want to do these things you are probably better off starting with the HumboldJSEmpty project, which already includes PureMVC, so check that out first.
After cloning this repository you can import the folder as an existing project in Adobe Flash Builder. This may not be needed depending on your usage scenario. Actually building the library is done by running the ant build script in the build folder.
Note that you may need to update the FLEX_HOME path in the ant build.xml