Mobile-first design means building apps and websites with phones in mind from...

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 with phones in mind from the start. It’s all about speed, so pages load fast and keep you moving. Buttons and menus are big enough to tap easily, even on small screens