From N Gauge wiki
Bookmarks is a feature that has been added onto the forum. It allows a user to add a thread from the forum and bookmark it so it's easier to find in the future.
How to add a bookmark?
To add a bookmark, go into a thread and look for the 'Add Bookmark' button just above the title of the thread. Once added, this will appear in a section on your Profile, not visible to other members.
Accessing and deleting bookmarks
You can access your bookmarks through the 'Profile' button at the top of the forum. A list of your bookmarks will appear on your screen and you can then select which ones that you wish to delete by using the check boxes to the right and then clicking 'Delete Selected Bookmarks', or you can seleect which bookmark that you'd like to view.