Talk pages (also known as discussion pages) are the pages that are attached to each normal page, and reservered for comments about that page.
When you just want to add a comment to a talk page instead of edit the whole page, you can click the + tab rather than the edit tab. This will allow you to put a subject / headline on your comment, which will also make your post show up in the table of contents.
The image on the right shows the link to the talk page of the homepage, but also pages that do not yet have any content already have talk pages. You could, for example, write your ideas on how the story should develop on the talk page of a chapter that does not yet exist (the red lines on the Overview page).