"Mark unread" does exactly what its name suggests. For example, if someone were to find a thread through "Show unread posts since last visit" and decided they wanted to save it for later reading without forgetting it, they could click "mark unread" so it reappears in the "Show unread posts since last visit" section. The only true way to "unmark unread" would be to just view the thread. It doesn't change the thread in any significant manor, it just simply allows users to mark topics for future/later browsing.
