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

https://www.wall-bookmarkings.win/mobile-first-design-means-building-websites-and-apps-to-work-great-on-your

Mobile-first design means building apps and websites with phones in mind first. It makes things load faster, fits buttons and links so they’re easy to tap, and keeps navigation simple so you find what you need without hassle