From Google Webmaster Central Blog:
"Today we’re announcing that after a year and a half of careful experimentation and testing, we’ve started migrating sites that follow the best practices for mobile-first indexing.
To recap, our crawling, indexing, and ranking systems have typically used the desktop version of a page's content, which may cause issues for mobile searchers when that version is vastly different from the mobile version. Mobile-first indexing means that we'll use the mobile version of the page for indexing and ranking, to better help our – primarily mobile – users find what they're looking for.
We continue to have one single index that we use for serving search results. We do not have a “mobile-first index” that’s separate from our main index. Historically, the desktop version was indexed, but increasingly, we will be using the mobile versions of content.
We are notifying sites that are migrating to mobile-first indexing via Search Console. Site owners will see significantly increased crawl rate from the Smartphone Googlebot. Additionally, Google will show the mobile version of pages in Search results and Google cached pages."
Developer guide here: Mobile First