Mobile-first indexing means Google's crawler primarily crawls + indexes the mobile version of every page. The desktop version is used only when no mobile equivalent exists. Google completed the rollout to nearly 100% of indexed sites by July 2024.
Practical implications: anything missing from the mobile version (hidden text, simplified navigation, fewer products) is effectively invisible to Google, even if it's present on the desktop version. Responsive design (one URL serving the same HTML to both mobile and desktop, with CSS adapting the layout) is the safest pattern.
Common pitfalls: using display: none to hide content on mobile (Google still sees it but ranks it lower if it considers the content "buried"); serving different structured data; breaking lazy-loaded images.