Dont Quote Me...but,
I believe that they refer to the number of posts and active posts.
The closed envelope being one with less than a certain # of posts or one that hasn't been posted to in a long time. Most of the threads at the bottom of the page have a closed envelope. The open yellow means certain # of posts and suggests that there have been recent posts, therefore it is still considered active. The orange one means that there have been many posts to that thread. The arrows take you to the last post in a thread since your last visit.
The dot means that you have posted to that thread.
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