Mobile-first design means building apps and websites to work smoothly 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 smoothly on phones first. It keeps things speedy, so pages load fast. Buttons and menus are easy to tap, with plenty of space for your fingers