Topics (Mark all read)

Hi, I ran across the above on the front page of ESL, General English Grammar Questions. What does it refer to? Thanks.
Give it a try and see what happens. Then let me know; I've never tried it.

Presumably, if you do not mark them 'all read', there is a button that will call up the unread ones for you. I know that the ones I read are un-bolded in the forum menus, but I've never marked anything read...because I might want to read it.
Thanks, Mister.