On April 21st 2015, Google changed their search algorithm to boost the search rankings of mobile friendly web pages. This means that pages that are legible and usable on mobile devices (phones, tablets, watches, etc) will rank significantly higher in mobile search results than those that are not mobile friendly. This applies to individual web pages and not the site as a whole.
Google has created a mobile friendly test tool for you to test whether your web pages are mobile friendly or not. If your site is not mobile friendly then you might want to think about changing it so that it is. More and more people are accessing the internet through mobile devices and not being visible in these search results is a hindrance to your online visibility.
How do I make my site mobile friendly?
There are two different ways to do this. The main way, which I would recommend, is to make your site responsive. This means that it is created in such a way that it adapts to whichever device it is used on and looks good on mobile, tablet or desktop. The second way is to make a mobile only site. Google can then be informed to index it seperately (for example, at m.yoursite.co.uk instead of www.yoursite.co.uk) or the content can be served dynamically based on what format the user needs.