What is a mobile-first website?

In practice it is a build order. Styles are written for the narrowest viewport as the default, then wider layouts are added as explicit enhancements. Working that way forces content priority decisions early, because a small screen genuinely cannot show everything at once, and it usually produces a lighter page than starting wide and hiding things.

The mistake is treating it purely as layout. A mobile-first site also has to be usable with a thumb — tap targets large enough to hit, forms that trigger the right keyboard, and no interaction that depends on hovering. And it has to be tested on a mid-range Android phone on a slow connection rather than a narrow desktop window, because that browser window is not going to reveal the performance problem.