Google's policy since 2019 of using the mobile version of a page as the primary source for indexing and ranking — what's on your mobile site is what Google sees.
Mobile-First Indexing in plain English
Mobile-first indexing means Googlebot's mobile crawler is the primary version that determines what gets indexed. Whatever content is missing from your mobile rendering — hidden in collapsed sections that don't load, behind "Read more" buttons that never expand, in tabs that JavaScript fails to render — is content Google doesn't know about, even if it exists in the desktop HTML.
The transition was announced in 2016, started rolling out in 2018, and completed in 2023. By that point, every site Google indexes is judged by its mobile rendering. The rare exceptions are sites that explicitly mark certain pages as desktop-only, but for any standard ecommerce store, mobile is the source of truth.
Common gotchas: structured data that only renders on desktop, images with different file paths between desktop and mobile breaking image search, internal navigation that's hidden on mobile (making large parts of your site uncrawlable from the mobile entry point), and any content gated behind "View desktop site" links that real mobile crawlers won't follow.
The recommended setup is a single responsive site where the same URL serves the same content to all devices, with layout adapting via CSS. Separate m.example.com mobile sites still work but are deprecated and add complexity. If you have a separate mobile site, every desktop page needs a paired mobile equivalent linked via rel="alternate".
Why mobile-first indexing matters for ecommerce
For ecommerce stores, this is existential. If your mobile product pages have stripped-down descriptions to save space, those stripped-down descriptions are what Google indexes — not the rich detail on desktop. If reviews load via JavaScript on mobile but JavaScript fails on Googlebot's render, those reviews don't exist for SEO. Every claim in your desktop product page needs to be present and crawlable in the mobile rendering or you've lost it. Test your mobile rendering with Google's Mobile-Friendly Test and URL Inspection tools regularly.