Singles and Unique Pages are pages on your site that have a unique design or serve as a landing page for other content.
You can find this content by going to Entries > Singles and clicking on the page you wish to edit. It is not possible to create a new page in this section. To create new pages you should go to the correct channel of content, such as blog, books.
The unique pages you can edit here include:
The fields for the entries are self explanatory.