The default CSS classes 'header segment' apply a margin above the main 'H1' heading to create white space between the primary navigation menu bar that runs along the top of the page, and the start of the main page content.
The standard layout structure uses a two column stackable grid inside a container which adds white space according to the view size: mobile or desktop. On larger screens, the sub navigation column appears on the right side of the page, and collapses below the main content when viewed on a smaller screen e.g. a phone.