Simply put, Google intends to reward websites that are responsive that offer a good user experience when viewed on a mobile device
; conversely, websites that are not mobile-friendly will be ranked below those that are Responsive (this applies to searches on mobile devices).
As 60% of all web searches are now via a mobile device (phones and tablets have overtaken desktops as the primary search device)
it follows that mobile-friendliness would increasingly factor in Google’s page rank prioritization. Don't be alarmed. There are a surprising amount of high profile websites that have not updated their web presence to reflect this change. Even the bbc only recently updated their website to reflect the shift towards viewing web content on mobile devices.