When you place this code on a form with tabs and open it in the desktop browser, you will notice nothing. But on a mobile device with a screen width less than 800px you will see the following results.
This is one of the implementations of a hamburger menu. Special thanks to Glenn Smith, whose code I took as a basis. You can find much more on the Internet.
If you want the menu to be visible in the desktop browsers, you can play with max/min width OR completely remove the first media query and the @media … wrapper for the second part.