Google has been rolling out lots of changes to its services and products, and also its guidelines that impact mobile. Many of those were believed to be its preparation for the new algorithm update.
On April 21st, 2015, Google began rolling out its mobile-friendly update algorithm that boosts the ranking of mobile-friendly pages on mobile search results. When those websites are pushed further up, websites that aren't mobile friendly are automatically lowered in priority.
The update is meant for web users so they can easily find high-quality and relevant results where text is readable without tapping or zooming.
The update affects only search rankings on mobile devices in all languages globally, and applies to individual pages instead of the entire websites.