About a year and a half ago, Google announced that it would be changing it's algorithm for internet searches to favor websites that were "mobile friendly". This decision came as that result of statistics that put mobile traffic at over 50% of all traffic for most searches. Obviously Google's goal was to ensure that results for those mobile searches were sites that would perform adequately on those mobile devices, but the algorithm was changed for all searches, not just those done on mobile devices. This means that if your site is not mobile friendly, then your search ranking is suffering. For many years, sites had separate "mobile" sites that were simpler versions of the full site. This also hampers your site for searches on mobile devices, as these separate (and simpler) mobile site are more brief and do not contain all the content and keyword phrases that the full site contains.
Fully Responsive Websites
Because of this, Google recommended that websites be a full site that is "mobile-responsive" instead of a separate "simpler" mobile site. A full mobile responsive site is one that automatically accommodates small screens (mobile devices) by changing the layout and text size according to the screen size. For instance, this site is mobile responsive. You can either look at this site on your smartphone phone, or you can simply re-scale your browser window to a smaller size to see the layout instantly change to accommodate the changes in screen size. This means that all your content is accessible on any device you use.