You may have received an email from Google Search Console (formally Google Webmaster Tools) informing you that your website is enabled for Mobile-first Indexing. Congratulations – Google is happy that your site is mobile friendly and this change will not adversely affect your search ranking.
If your website does not have a mobile version, it will still get included in the index, but may appear lower in the rankings than a website that is mobile friendly.
If your website has a mobile version, which includes mobile responsive, Google will now read and index the content of your website in ‘mobile view’.
Content: Make sure your valuable content is visible on both mobile and desktop views.
Images need to be optimised for mobile, high-resolution images take longer to download.
Speed: Check using your pages using Google page speed tool and fix any issues found.