It's an extension that makes GitHub pages full width: https://github.com/xthexder/wide-github/

Admittedly the usefulness has gone down a little bit in the last couple years now that GitHub themselves have made code diffs and some other things full width by default.

When I first wrote this I had just gotten a giant 4K display at work and was really annoyed I still had to scroll left and right with the page only covering 1/3 of the screen.