That's simple greasemonkey script that adds public trackers to every magnet link on page. That's way you can always get as much peers as possible for semi-dead torrents.
Just install this script to greasemonkey addon (firefox/chrome/opera) by copying it's contents, creating new script in greasemonkey addon and pasting script content's there.
This scripts does request to public trackers list on github that got updated everyday and expand each magnet link on page. Requests to github are cached by default.
You can see that script is working if you open javascript console. You should get messages tagged by "Magnet-Booster".