Your tab that says "Unread Posts" is actually showing "New Posts." New posts are posts that were posted since the last visit (plus whatever delay time your software sets). An unread post is one that has never been looked at by the user. This is sometimes tracked by seeing if or when the user clicked on the post's thread. I remember long ago using a forum that could track both.
Your tab also says "Undone Threads." I can't figure out what that is supposed to be. Undone usually means "incomplete" or "not finished," which implies that there is work left to be done on these threads.