Mobile-Friendly is the term Google uses to describe sites and pages that meet its criteria of mobile optimization. mobile-friendliness became a ranking factor in April 2015.
According to Google
?Mobile is changing the world. Today, everyone has smartphones with them, constantly communicating and looking for information. In many countries, the number of smartphones has surpassed the number of personal computers; having a mobile-friendly website has become a critical part of having an online presence.?