When a website is responsive, the layout and/or content responds (or, adapts) based on the size of the screen it’s presented on. A responsive website automatically changes to fit the device you’re reading it on.
Starting April 21, 2015, Google Search expanded its use of mobile-friendliness as a ranking signal. The change had a significant impact in Google Search results. Mobile responsive websites will have relevant, high quality search results optimized for their devices.
Is your site mobile friendly? Take the Google Mobile Friendly Test.