Mobile-first design means building websites and apps to work great on phones...

https://deansinspiringperspective.hexaforgey.com/posts/why-mobile-optimization-is-no-longer-a-one-time-project

Mobile-first design means building websites and apps to work great on phones first. It focuses on fast loading, easy-to-tap buttons, and simple menus that fit small screens. This makes browsing quick and smooth, even on the go