Prepare for mobile-first indexing

" Since the majority of users now access Google Search with a mobile device, Googlebot primarily crawls and indexes pages with the smartphone agent going forward.  Starting July 1, 2019, mobile-first indexing is enabled by default for all new websites (new to the web or previously unknown to Google Search). For older or existing websites, we continue to monitor and evaluate pages based on the best practices. We notify site owners through Search Console once they're seen as being ready. Since the default state for new sites is mobile-first indexing, there's no need to send a notification to new sites. "

What this means for your site 

Based on what type of site you have, here's what mobile-first indexing means you:

If you have this type of site...
Desktop only: Your site is desktop only and doesn't have a mobile-friendly version. No change. The mobile version is the same as the desktop version.
Responsive web design: Your site adjusts for screen size. No change. The mobile version is the same as the desktop version.
Canonical AMP: All your web pages are created in AMP HTML. No change. The mobile version is the same as the desktop version.
Separate URLs: Each desktop URL has an equivalent different URL that serves mobile-optimized content. This site type is also known as an m-dot site. Google prefers the mobile URL for indexing. To prepare for mobile-first indexing, follow our best practices.
Dynamic serving: Your site serves different content based on the user's device. Users only see one URL. Google prefers the mobile optimized content for indexing. To prepare for mobile-first indexing, follow our best practices.
AMP and non-AMP: Your site has both AMP and non-AMP versions of a page. Users see two different URLs. Google prefers the mobile version of the non-AMP URL for indexing. If your non-AMP mobile version uses dynamic serving or separate URLs, follow our best practices.