The goal is to create or generate some files in the user's app directory, so that he can make some uses of it. For instance, provide LESS files that uses end-user's LESS variables (eg. the user's custom bootstrap theme).
This gist is just explaining the method Marco Pfeiffer has found to make its (awesome) highly configurable LESS boostrap package.
In order to create some file in the user's app workspace, you need to create a build plugin with the registerBuildPlugin API. Then you need to get it called at least once during build. This is done by registering a sourceHandler on some file extension. Your build plugin we get called once per file matching the extension. For now only one build plugin can register as a handler for a given extension