The first time someone comes into the forum with the new software, it shows all of the posts as unread. (Or at least it did for me.) Instead of clicking on every single post to "read" it, marking them all as Read is faster. And yes, the gray font shows the read posts.
Another way is when you're wanting to leave the forum, click the down arrow in the "Forums" box in the upper left hand corner. You'll get a drop down menu and at the bottom of it click on "Mark Forums Read".