Mobile-first design means building apps or websites to work best on phones and...

https://www.newpelican.com/articles/why-mobile-first-design-has-become-the-standard-for-modern-digital-services/

Mobile-first design means building apps or websites to work best on phones and tablets. It focuses on speed, so pages load quickly even on slow connections. Buttons and menus are big and easy to tap with your fingers