Hi Ju,
When you click on "Reply" to a topic you normally type in to the "Message" section.
If you take a look slightly above that you will see "Subject". This box can be changed by the member as it represents the subject of the reply. The subject of the thread remains the same though.
So, basically you can have an overall subject for the thread, and each reply can have a sub-subject name.
I hope this clarifies it for you
