It's where people have given you good rep for your posts.In other words they've liked your posts.you can go into your user control panel and see which posts you were left rep for and from whom if they signed their name.
Actually, you're only able to see the names of people who left rep for you if you are a vip, manager or moderator on the site. Regular members can only see that they were given rep and how many points they have. Hope that helps you. Sometimes people choose to leave a comment along with the rep, and sometimes they just hand it out and keep going. Also, if you were given rep for a post in a thread that no longer exists for whatever reason, it will show n/a for the title of the thread.