User interface design or user interface engineering is the design of user interfaces for machines and software, such as computers, home appliances, mobile devices, and other electronic devices, with the focus on maximizing usability and the user experience.
The "front-end languages" live in the browser. After you type an address into the address bar at the top and hit the enter/return key, your browser will receive at least an HTML file from the web server. That file will likely tell the browser to request a CSS file and a JavaScript file as well (probably many more than one, but we'll keep it simple).
Backend development empowers the web. Yet it does it modestly, without fanfare-allowing people to browse their favorite sites without even knowing about all the work put in by the backend developer or team.