Out-of-the-box Discourse enables “title” notifications, this feature will “re-write” the browser title to denote how many new items your current page has.

User may opt to receive “favicon” notifications by selecting Show new / updated topic count on browser icon in user profile, when this is selected the notifications will be super-imposed on the browser icon.

This feature operates in 2 modes:

First mode is if you are watching topic list.

Or in the icon mode:

:alarm_clock: in topic list you should expect that the count in the blue box will always match the count in the title or icon.

Second mode is for watching a topic

By placing your tab on a topic page and then kicking tab to the background, we will start counting all new posts in the topic.

:alarm_clock: As soon as topic gains focus the count will go down to 0

Feature use cases:

  • Quickly tell if someone posted something new on a particular topic.
  • Quickly tell if you have new or unread posts on a category.
  • Quickly tell if there has been any new activity in the site since you last looked.
  • Unlike unread notifications this feature can be tapped by anonymous users and allows very high fidelity of notifications.

Source: https://meta.discourse.org/t/how-does-title-and-favicon-notification-work/33508