Mobile-first design means building apps and websites primarily for phones and...

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

Mobile-first design means building apps and websites primarily for phones and tablets. It makes everything load faster, so you don’t have to wait. Buttons and menus are easy to tap with your fingers, not tiny little clicks