The scenario

1. We want a red background in mobile.
2. We want a green background in tablet and desktop.

Is this mobile-first?

body {
  background: green;
}

// This applies from 0px to 480px
@media (max-width: 480px) {
  body {
    background: red;
  }
}

No.

Is this mobile-first?

body {
  background: red;
}

// This applies from 480px onwards
@media (min-width: 480px) {
  body {
    background: green;
  }
}

Yes.

Conclusion

Developers often code desktop-first rather than mobile-first.

I was like that until I learned tailwindcss.