Google systems are currently testing websites for critical mobile usability errors. These errors are normally related to “older” websites that were built before the “responsive mobile website” technology.
If your website is not responsive this will affect how mobile users are able to experience your website. And Google has now advised that these pages will not be seen as mobile-friendly by Google Search, and will therefore be displayed and ranked appropriately for smartphone users.
One way to check if your website is mobile friendly is to look at the site on your phone and your Home page and you should have a menu that’s easy to read and no scrolling or pinching and pulling required.
Here is a tool for you to check your website for mobile friendliness.
Also a good read on “Why Make A Website Mobile Friendly” at this link.