On the bottom left is a link to bookmark a thread. You can access bookmarks via the control panel.
Overview panel>Manage BookmarksFYI the other link labeled subscriptions would be if you wanted to manually subscribe/unsubscribe to topics for email notification. You can also do this for forums, e.g if you subscribe to a forum you'll get a notification when a new topic is created. When you post in topic you can be automatically subscribed to it for email notification. The default I believe when you register is
not to notify you, you can change the default setting under
Board preferences>Edit posting defaults