Inline Forum Notifications are notices that appear at the top of the thread list view (forum/view) or currently viewed thread (thread/view) page when new content has been added.
These can be enabled individually for new threads and new posts through the Control Panel at Plugins > Forums. By default, this feature is disabled for new networks.
How the notifications work
When a user visits a thread list (forum/view) page, for example, the system watches for new activity within the forum. If a new thread is added, the system performs an Ajax call to add a message to the user to the top of the page. The system will check three times for new activity.