Mobile-first design means creating apps or websites that work great on phones...

https://www.seo-bookmarks.win/mobile-first-design-means-building-websites-and-apps-with-phones-in-mind-first

Mobile-first design means creating apps or websites that work great on phones first. It focuses on speed, so pages load fast and save your time. The layout is easy to tap with your fingers—no tiny buttons to miss