Back on April 21st 2015 Google launched a mobile friendly algorithm, this started using mobile-friendliness as a ranking signal on mobile searches .Therefore if your website did not meet the mobile friendly guidelines you could not be found in the Search Engine Results Pages (SERP’s) for users searching on their mobile devices (smart phones & tablets) however if Google was confident the page provided great content for the query it could still show in the SERPs.
Google announced on 16th March, 2016 that beginning in May they will start rolling out an update to this algorithm, which will “Increase the effect of the [mobile friendly] ranking signal”.
As always Google have been very vague about the updates relating to their ranking signals however we do know if you do not have a mobile friendly responsive web site, and you did not feel the effect of the mobile-friendly algorithm update last year or maybe you do have a mobile site but it is not responsive, you will almost definitely notice an impact from the update starting May 2016.
Yes, you read that correctly, having a mobile site alone is not enough anymore to qualify for the mobile friendly update. It must be responsive.