Mobile First Indexing

Back in November 2016 Google announced that experiments had begun to make its index ‘mobile-first’. In simple terms, this means that Google’s algorithms will, at some point in the near future, treat mobile content as primary content, and not desktop-optimized content as it has done historically, when indexing and ranking pages in its SERPs.
It should be made clear however that this update is not about rewarding or penalizing sites based on how ‘mobile-friendly’ they are, and will only really have a significant impact on web properties that have different versions for mobile and desktop.

For example, let’s imagine a website that has two different versions of a homepage – one optimized for mobile and another for desktop – but the mobile version contains less content, is slower to load, and generally provides an inferior user experience to that of the desktop version. 

With the mobile-first update, the mobile version will be treated as the primary page, so regardless of how good the desktop version is, Google’s only going to look at the mobile version, which will naturally impact SEO.

Considering up to 60 percent of searches now come from mobile devices, mobile-first is a sensible and somewhat inevitable move on Google’s part, as it will ultimately provide a better experience for searchers. However, as with any algorithm-related change, the shift towards a more mobile-focused web brings with it a number of concerns for site owners. For those with questions relating to how this update might affect SEO, and what actions should be taken to ensure stability throughout the transition, the following questions and answers should help clear things up.

My site is responsive, do I need to change anything?

Nope. If your site currently serves the same content and markup across mobile and desktop, you shouldn’t have to change a thing.

My site is not responsive, should I be worried?

Possibly. If your site is configured to serve different content and markup across mobile and desktop, it could have a negative impact on SEO. To ensure minimal disruption, Google recommends making the following changes:

• Make sure to serve structured markup for both the desktop and mobile version. Sites can verify the equivalence of their structured markup across desktop and mobile by typing the URLs of both versions into the Structured Data Testing Tool and comparing the output.
• When adding structured data to a mobile site, avoid adding large amounts of markup that isn’t relevant to the specific information content of each document.
• Use the robots.txt testing tool to verify that your mobile version is accessible to Googlebot. Sites do not have to make changes to their canonical links; we’ll continue to use these links as guides to serve the appropriate results to a user searching on desktop or mobile.

Information Reference : http://www.browsermedia.co.uk/2017/06/23/mobile-first-indexing/