As we noted earlier this year, today’s the day we begin globally rolling out our mobile-friendly update. We’re boosting the ranking of mobile-friendly pages on mobile search results. Now searchers can more easily find high-quality and relevant results where text is readable without tapping or zooming, tap targets are spaced appropriately, and the page avoids unplayable content or horizontal scrolling.
http://googlewebmastercentral.blogspot.com/2015/04/rolling-out-mobile-friendly-update.html