Mobile-first design means creating apps and websites that work great on phones...

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

Mobile-first design means creating apps and websites that work great on phones first. It’s faster to load, with big buttons and easy-to-tap features that fit your screen. The menus are simple, so you can find what you need without fuss