As I found out that most questions are not stupid, I will ask about this function. I have not pressed it yet because I have not read all forums as there is usually something new to be read. What is it for?
Usually, when someone comes into the forum, they want only to read the posts that they haven't already read. Since the forum keeps track of what you have read, that is very easy. However, not every forum thread is likely to be interesting to everyone. Yet, if you don't click into them, they will be flagged as being unread the next time you come into the forum. So, one good use of this feature is to click it after you have read every new post that you're interested in reading. It will indicate that every post has been read so the same threads that you declined to read last time won't be presented to you when you come back, unless someone posts something new in them. In this forum, I try to read every post, since it's my forum. However, in other forums that I participate in, I use it a lot. Especially if I log in only once every few days, the shear number of new posts can be overwhelming, so after I have read the ones that I think might be interesting, I'll click to mark the rest of them as having been read. If I come back later that day then, I can see just the ones that are new.
Thanks, Ken! Now I know everything again! I probably will not use it as I like to read new stuff even when not especially interested in the topic. I can always learn new things and ideas!
For anyone who may not have noticed, there are a couple of ways to find posts that you haven't read. If you simply display the forum menu, the sub-forums with new posts in them will be highlighted, with a "New" tag. Or you can click the hyperlink that says "New Posts" near the top of the page, and all of the new posts will be presented.