Over the last years Google gradually increased the importance of mobile optimized websites. In 2014 Google started indicating non-functional pages in mobile search results and later that year added the label “mobile-friendly” to optimized pages. This year Google made the next step and now uses mobile-friendliness as a ranking signal. Summed up: Websites, that have been optimized to be viewed on a mobile phone are ranked higher in mobile search results (Desktop and tablet search results are not relevant).
Test on mobile optimization
To get an initial overview of the current status of your website, you can test your site here.
Answers to FAQs can be found here.