Just like your computer, all of your site's content is stored in a series of folders and files (called Pages), which are listed in the Pages tab of the Admin.

The hierarchy and order of the Pages are important: they are used to automatically create the Main Navigation in the header.

Pages placed at the top-level of the structure (or root) are used to create standard menu items like Home & About Us in the screenshot above. Folders create dropdown lists which display all the pages they contain.
There are two exceptions to how the Main Navigation treats Pages: Contact Us & Blog. Any Page usign the Blog template will be displayed as a standard menu item (a simple link), instead of a dropdown. This is becaues, although the Blog page has children, listing them all in a dropdown would be excessive.
Contact Us on the other hand is treated like any other Menu, but adds styling to turn it into a Button.