understrap/node_modules/grunt-contrib-sass/docs/sass-overview.md

Sass is a preprocessor that adds nested rules, variables, mixins and functions, selector inheritance, and more to CSS. Sass files compile into well-formatted, standard CSS to use in your site or application.

This task requires you to have Ruby and Sass installed. If you're on OS X or Linux you probably already have Ruby installed; test with ruby -v in your terminal. When you've confirmed you have Ruby installed, run gem install sass to install Sass.

Note: Files that begin with "_" are ignored even if they match the globbing pattern. This is done to match the expected Sass partial behaviour.