Mobile-first design means building websites or apps starting with the phone...

https://www.toro-bookmarks.win/mobile-first-design-means-creating-apps-and-websites-that-work-great-on-your-1

Mobile-first design means building websites or apps starting with the phone experience. It’s all about fast loading times, easy-to-tap buttons, and simple menus that make getting things done quick and painless on a small screen