From Google Developers

The Mobile-First Index is the name given to Google’s recent algorithm update. This update states that Google will predominantly use the mobile version of site content for indexing and ranking within organic search results. Prior to this update, the algorithm primarily used the desktop version of the site’s content when evaluating relevance to a user’s query and how pages should rank. Now, with mobile-first index, Google will use the site’s mobile version when indexing and ranking for both mobile and desktop.

With the mobile index, Google will use the “smartphone agent” to crawl pages, as opposed to the standard Googlebot crawler it had previously used. Crawler, as it relates to Google, is defined as an automated program meant to discover and scan websites by following links from one webpage to another. These crawlers then rank the webpage based on the user’s query and various factors taken from the larger Google algorithm. The major shift with this algorithm is that Google will now treat all websites as if a user is searching from a smartphone rather than from a desktop.

Best Practices to Consider:
• If a site is responsive and configured properly for mobile, no changes should be necessary. Google has several tools that you can leverage to determine if your site is viewed as mobile-friendly
• Ensure links to all XML sitemaps are accessible on mobile
• Ensure that the robots.txt file is accessible on mobile and that no directives are blocking key mobile pages

For Mobile-Separate Sites:
• All metadata must be present and unique across each page, and match on both the mobile and desktop versions of the site
• Ensure proper canonicals have been implemented and match on both versions of the site
• Both the mobile and desktop versions should have the same content. This includes all live assets on-site (e.g., text elements, images, videos, etc.)
• Verify that the server has enough capacity to handle an increase in crawling of both mobile and desktop versions of the site
• Any structured data or schema markup that is present must match on both the desktop and mobile versions of the site
• For sites with multiple language versions, ensure all hreflang tagging has been properly implemented and that the mobile URLs and desktop URLs have their own specific hreflang tags