it's most useful for fantasy or sci-fi stories done in original worlds, to make sure that everyone is on the same page when writing posts. it's where you can put things about technology, magic, monsters/aliens, religion, history, ect. it's basically just a place to collect important background...
You have to edit the first post in the thread. When the small edit box pops up, click "more options," and the edit page there will have the title as an editable field.