In a nutshell, Google now prioritizes responsive websites which are sites using designs that respond to the size of any given screen. This is different from mobile-friendly sites that can actually be two separate sites that look similar.
How do you know if your website is responsive? Easy - if you're on a computer, just change the size of your browser window to a smaller sized window. If the website adjusts with the browser, then it's responding to the new size. If it just crops out the site and the website doesn't change to the new size, then you have a non-responsive website.