What is watching a topic?
Is an option at the bottom of a posting screen.
You can set it up to be done automatically for every thread you start by selecting that option in your profile. Otherwise you will need to chose the option in each thread.
What watching does, is email you if that thread has been replied to.
For example, you are writing a thread about a topic that interests you and you have finished. You can check on notify me when a reply is posted.
This will send you an email when there are unread replies to the thread.
You can choose to cancel this option at anytime by clicking on Stop watching this thread.