There was a long thread about this topic which was started on 9/30/01 and titled “Green, yellow, and thumbs up/down”. You may wish to read that thread for more detail. But to summarize:
In addition to the “view last 24 hours/last 2 days/etc.” pull down menu that you found, when you view an index of the posts, the ones whose icons are green indicate that that particular post, or one of its replies, is new since your last visit.
Also, those threads which have most recent activity are shown at the top of the index.
So, while there’s no way to limit and only see those posts since your last visit, the “Green” icons mark basically the same thing. Plus, since they don’t hide the older posts, it allows you to read those again if you need some context to understand the newer ones.
Please let me know if you have any other questions.