It's a browser plugin that someone shared a while back.
Someone was so upset over red rep they had received that they were determined to find a way to see who had sent it to them, and it turns out it was possible to see who sent you rep all along. If you do "View Page Source" you will notice something like this after a rep comment: reputationbit_2096429_2899
The numbers after the second _ are the person's forum ID.
Then you pull up this person's profile by referencing their forum ID, like thus:
http://forum.gsplayers.com/member.php?2899
So we can see the person who sent me this rep was Stanley Burrell. It was green rep because Stanley loves me.
Someone took all of the IDs of everyone (at the time so it's probably outdated now and won't work for profiles made afterwards) and made a plugin so whenever you view your rep page it will list who sent you the rep.
You can still do it the old fashioned way without the plugin of course.