The sidebar that you see on desktop and laptop computers is transformed to a top navigation bar when the screen size is reduced to 700px or less. This is done by using media queries.
Another media query for screens that are 500px or less has been added, which will vertically stack and center the navigation links.
The sidebar has a fixed position, and to prevent it from covering the contents (e.g. this text), the class .content needs a left margin the same width as the sidebar. It would be best to extend this class down the page and place all the contents inside it. The width of the navigation is limited on this page by width settings on an outer container. This can be changed - or removed.
Resize the browser window to see the effect.