Mobile devices are the future of internet browsing and Google is making its move and leaning towards that trend — it has announced that it is experimenting on a "mobile-first" index.
Google’s search engine algorithms will “primarily” use the mobile version of a site’s content to rank pages from that site, to understand structured data, and to show snippets from those pages in its result."
That said, the company made assurances that its search index will continue to be a single index of websites and apps. In short, Google isn’t fragmenting its search index.
What this means for publishers
Lucky for publishers, Google has published a list of guidelines to follow. For the most part, websites whose mobile and desktop version share the same content will have nothing to worry about. Those that don’t should probably take a look at the guideline.
Google also recommends site owners who’ve only verified their desktop site in the Search Console to add and verify the mobile version of the website as well. Websites which are desktop-only will continue to be indexed just fine. However, without a mobile version, those sites will most likely rank lower than a website with a mobile version when Google makes its mobile index the de facto index.