Have seen this occasionally, whether logged in or not, on a couple of forum posts. No results from the amazing ARFCOM search engine. What does it mean? Is there a location/thread for learning more about the site with regard to things like this?

The last post you read.

Kyuss is correct. A bunch of us were having issues last year with the site and getting to the last read post in threads. We would hit the green arrow to go to the newest post in a thread and it would take us to the first post in the thread instead. Gaotboy worked on it and found if he turned on the function of the green line at the last read post in a thread, then that fixed those if us having the last read post problem. Since most things are site wide changes and not for an individual user, everyone now sees the green line.