Brunch is a front end build system that lets you get stuff done with little configuration. Brunch compiles, concats and (optionally) minifies your scripts and styles. It can also package JavaScript files into AMD or CommonJS modules. Brunch automatically applies plugins in the correct order to the right files - so with the right plugins, a .coffee file would be converted into a .js file and automatically minified, with no explicit setup necessary.
Compared to other build tools such as Grunt, Brunch is simple. Grunt's makes very few assumptions about your code or build process, and as such is more flexible in terms of what it can do. However, this flexibility comes at the cost of verbosity.