Mobile-first design means building websites and apps starting with the phone...

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

Mobile-first design means building websites and apps starting with the phone experience. It focuses on fast loading, so you’re not stuck waiting. Buttons and menus are big and easy to tap with your finger, making everything smooth and simple