This allows you to be taken to the first post in a thread since the last time you visited.

There are 2 different ways to access the first unread post in a thread.

Forum List Location

If you are in the forum list location where you can view the list of threads, look for the blue circle to the left of each thread title with white arrows pointing down. Click this to be taken to the first unread post for that particular thread.

Thread Page

If you are already currently in a thread, scroll up to the top of the window and click on View First Unread in the gray bar above the first post listed on the page. This link will also have a blue circle with 2 white arrows pointing down to the right of the link.