To subscribe you click on the "Subscribe" link at the bottom of a thread...
The point is to get an email update every time someone posts on the thread. Some folks do it if they don't want to post on the thread but still want to know when people post on it.
You can also click on "show your subscribed topics" on the index page to see a list of the topics you subscribed to, if you don't want to check your email. This is the only thing I use to keep track of a topic.