Mobile-first indexing is an approach used by search engines to prioritize the mobile version of a website for ranking and indexing rather than the desktop version. Since more users now access the web through mobile devices, search engines like Google have shifted to mobile-first indexing to provide a better and more relevant experience for mobile users.
With mobile-first indexing, search engines crawl and evaluate the mobile version of a website first when determining its ranking in search results. This means it's important for websites to be mobile-friendly, with content, images, and other elements optimized for mobile devices. Websites that aren't optimized for mobile could see a drop in their search rankings.