When you're on a page that's a list of threads, the far left column is a bunch of envelopes:
Yellow open envelopes are threads with new posts since the last time you viewed them.
Red open envelopes are threads with new posts since the last time you viewed them that have over 15(?) replies.
Blue closed envelopes are threads you've read since the last post was made.
Blue closed envelopes with little lines at the top are threads that have over 15 replies and that you've read since the last post was made.
Any envelope with a little black dot on it is for a thread where you have made at least one post.