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

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

Mobile-first design means building apps and websites with phones in mind first. It makes pages load faster, so you don’t wait around. Buttons and menus are big and easy to tap, no pinching or zooming needed