There are many things that are going on with forum statistics and currently active users. You have to know the specific forum or website to know this though. So there cannot be a better answer given until you are more specific.
If you were to visit a forum site, such as a career forum site for example, one can navigate through the site and see particular sections that would guide you about the site's activity. This is usually marked by a section that says: "What's going on". Under this, you can find "Forum Statistics", where you can see stats from the site such as the number of currently active users.