Mobile-first design means building a website or app with your phone in mind...

https://www.newpelican.com/articles/why-mobile-first-design-has-become-the-standard-for-modern-digital-services/

Mobile-first design means building a website or app with your phone in mind first. It loads fast, fits nicely on small screens, and makes buttons easy to tap. Navigation stays simple so you find what you need without fuss