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

https://lanesniceblog.scriblorax.com/posts/why-does-my-website-look-fine-on-desktop-but-messy-on-my-phone

Mobile-first design means building websites and apps with phones in mind first. It makes pages load faster, so you’re not waiting around. Buttons and links are big enough for your fingers, not just a mouse pointer