The Duck is correct. BOLD type on a subject line means there are posts to the thread that you havent seen since your last visit.

When you start a new thread - after you post it, you go back to the forum but it's not bold because you have already seen it. Once you leave the forum, then come back, it will be bold if someone has replied to it (In other words, it will be bold if there are replies that you have not seen)