These days, most Google searches are performed by mobile users.
In the past, Google crawling, indexing and ranking have been based on the desktop version of your website; but when the Google roll-out of mobile-first indexing reaches you, results will be based on the mobile view of your website. Though some sites have switched prior, the wide-scale roll-out began in March 2018.
If you signed up for Google Search Console (previously Webmaster Tools), you will receive an email notification when your site has been switched to mobile-first indexing. Google says the roll-out thus far is only for sites that “follow the best practices for mobile-first indexing“.
If your website has a responsive design nothing will change – as long as the same content exists for desktop as for mobile. If you have content displaying on desktop that doesn’t display on mobile, or vice versa, make sure your mobile content has been optimized for Google.
If you have separate mobile and desktop urls, the mobile urls will begin to appear in results instead of your desktop urls. And, now your mobile site content is what will be ranked.
So, there are a couple of recommendations. First, do a site audit and fix anything not working properly, because the Google crawl rate will increase once your site has switched. If you have a separate mobile version, optimize it for ranking, or instead make your site responsive so mobile & desktop share same urls and same content. Check your Analytics historical data to make sure your rank is where you expect it, and continue monitoring regularly, adjusting as needed. In competitive markets where much of your business depends on Google results, you may want to hire an SEO expert if you experience declining traffic that results in lost business.