What are the numbers in the tabs?

Very frequently, a number will appear next to the forum topic in the tab of my browser. It looks like this: (2). What does it mean? I always thought it meant something about that forum was changed, but I’m not too sure.

Perhaps there have been two new messages since you’ve last read the page?

The number states how many new replies have been posted since you’ve last visited the topic.

but that’s impossible. I created a new topic not too long ago and no one has posted to it. Yet it had (2) on the tab.

Correct me if I’m wrong, but I think it’s the number of topics that have new, unread replies whilst you were browsing the current topic. For example, a (2) means that two other topics gained new replies while you were reading this.

That makes more sense. Thank you.