A responsive website means that the website responds to the size of the screen. A good mobile-friendly responsive website will look different on a desktop or tablet than on a phone. A good mobile-friendly design will have text that is large enough to read on a phone and links that are spaced far enough apart that the user can easily click them.
In April 2015, Google started to rank websites that are mobile-friendly higher than those that are not. For some companies, this meant that their website went down in Google’s search rankings. Here are some articles you might want to read.
You can also test your website here to see if Google considers it to be mobile-friendly.