The Media Tray is the horizontal tray located below the page(s) being worked on. It displays either imported photos or the pages of the section you are working on.
To toggle between photos and pages, simply click on the Photos/Pages tabs located in the bottom left corner.
In the Photos tab, you can click on Manage Photos to expose more options like Add Photos, use photos from another section, filter photos by tag (if applicable), hide used photos, and sort photos by date or filename.
In the Pages tab, you can get a bird's eye view of all pages in the current section. You can also jump to any page by clicking on the thumbnail of the desired page.