This is a Chromium/Firefox-based extension that "explanifies" xkcd comics. When you click on the extension, then click explanify, it brings you to the explanified version of the comic on Explainxkcd. This Github version is the development version of the extension and may be riddled with bugs and other problems.
You could find most of the browser specific files in their respective folders. For all Github related stuff such as, code of conduct, contributions, readme, etc, those are found in the /.github directory