Mobile-first design means you literally design and build the mobile version of your page before the desktop version. The mobile layout isn't an afterthought or a squished-down desktop page — it's the starting point. You then progressively add complexity for larger screens using CSS media queries that add rather than override.
This isn't just a methodology preference. Over 60% of web traffic is mobile, and for many industries (especially B2C, local services, e-commerce), it's 70-80%. If your landing page was designed desktop-first and then "made responsive," the mobile experience is almost certainly compromised: buttons too small, text too dense, horizontal scrolling issues, forms that are painful to fill out on a phone.
Mobile-first vs. responsive: the real difference
A responsive page adapts to screen size. A mobile-first page was conceived for small screens. The difference shows up in content decisions: a desktop-first responsive page tries to fit everything from the desktop layout onto mobile. A mobile-first page asks "what's essential?" and only adds extras for larger viewports. That constraint produces better, more focused landing pages at every screen size.
Practical test: check your analytics for mobile conversion rate vs. desktop. If mobile converts at less than half the desktop rate, your page probably isn't truly mobile-first. Google also uses mobile-first indexing — it evaluates the mobile version of your page for rankings, regardless of what your desktop version looks like.