If you go just above your post, there should be an option called "thread tools" with a triangle. If you click the triangle, a box opens, and an option is to "subscribe to this thread". If you click it, you've subscribed to it. Posting in the thread will also mean that you've subscribed to it.
Once you've subscribed, you can go to the option "user cp" which is your control panel. It will show you a list of the threads you've subscribed to, so you can see if they've updated.
Very useful if you want to keep an eye on a thread, and discover that you can't find it! Subscribing to a thread is how you keep the threads you are interested in reading in one location, in your user cp.