If you click or tap on unread, it explains what it is, like so:
Your unread topics appear here.
By default, topics are considered unread and will show unread counts if you:
- Created the topic
- Replied to the topic
- Read the topic for more than 4 minutes
- Or if you have explicitly set the topic to Tracked or Watched via the notification control at the bottom of each topic.
Visit your preferences to change this.