This file configures a Grunt task called "cssmin".
It minifies the intermediate, concatenated CSS stylesheet which was prepared by the concat task at .tmp/public/concat/production.css. Together with the concat task, this is the final step that minifies all CSS files from assets/styles/ (and potentially your LESS importer file from assets/styles/importer.less).
For additional usage documentation, see grunt-contrib-cssmin.
An example of this file can be found in our sample app repo, here.