And just to expand on that a bit - in order to qualify as a "hot" thread, a thread must have either or both of the following: 50 replies in the thread, and/or have been viewing 500 times or more.
Also, note that for both the blue and red threads, there are two types of envelopes. One is a closed envelope which means there are not posts unread by you. The other is an open envelope with papers sticking out which means there are some unread posts in that thread.