Mobile-first design means building apps and websites to 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 building apps and websites to work great on phones first. It prioritizes fast loading so you’re not stuck waiting. Buttons and menus are bigger and easy to tap—no pinching or zooming needed