Google is currently (at the time of writing) preparing to switch from a Desktop to a “Mobile First” search index.
What this means is that to ascertain if your website meets the criteria needed to be ranked, Google will be making that decision based on how your website appears on a mobile as opposed to a desktop. This change has come around because there are now more searches done on mobile devices instead of desktops.
To be mobile friendly you need to have a responsive website design, one that changes to accommodate different screen sizes and not a different m. mobile website. A different mobile website, (usually m.yourdomain.com) which were all the rage when google first started looking at “mobile readiness” does not contain all the necessary optimisation coding to help you qualify for a search result.
Also mobile only versions usually do not contain enough “Topical Depth” for Google to understand the topic and match it to the intent of a users search query.
According To The Google Webmaster Blog: