Mobile-first design means building apps or sites to work great on phones before...

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

Mobile-first design means building apps or sites to work great on phones before anything else. It keeps things fast to load, so you don’t get stuck waiting. Buttons and links are big and easy to tap with your finger