When you start a thread.....you will automatically be set to follow it's progress - "watch" it
This, means you will receive inline and email notifications if someone replies or quotes your post etc
You can choose to unwatch your thread if you wish......hence the "unwatch" tab
I do believe it means if you "click" on it you will not get any notifications when someone posts something on your thread. My screen says, "Watch" in the same place. If I "click" on it I will be notified when anyone posts.