Google announced in April that they were rolling out an update to their algorithm to include whether a site was mobile friendly and use this as part of the method of ranking websites on the search engine. With a huge amount of internet browsing and purchasing done through mobile devices, it was only a matter of time before Google started to rank sites accordingly based on the way they function on these platforms.
There is a website that you can go to and put in your URL to determine whether it is mobile friendly or not. A site can be programmed in such a way that it is or if the desktop version is not compatible then a stand-alone mobile version can be created. If your website was designed using WordPress, a free off the shelf content management system, then there are a number of plugins which can be added to your site to achieve this.