First of all, you're not slow
Sometimes people want to get notices about new posts in a particular thread but they don't have anything they want to say in it right then. So they type "subbing" because by posting ANYTHING in the thread, you are automatically subscribed to get notices of new posts.
You don't
have to do that, though. If you want to subscribe to a thread, all you have to do is click on the words "THREAD TOOLS" and a little menu will drop down. Choose "SUBSCRIBE TO THIS THREAD" and you will be all set.
Hope this helps!