This feature is found under forums/shortcuts/Posts you have not read.

It will display all the posts that you have not read. It's a really great feature (IMHO.)

To get started, you will have to mark all posts as read in each forum one time. This will get all the old posts cleared from the list. Do that under 'More options' (bottom of page) on each forum and click the 'Mark all read' button. This will remove that forum's posts from your 'Posts you have not read' list.

After that when new posts are added they will be displayed on your 'Posts you have not read' list until you view them.