On April 21st, Google will significantly increase the weight of “mobile-friendliness” as a ranking factor. Basically, if you’re website isn’t “responsive” or “adaptive,” you may be bumped down below other sites that are mobile-friendly in search results.
Mobile-friendly sites change shape to best fit the window-size of any device, from giant graphic design monitors to tiny, 5-year-old smartphones. They also tend to lean toward HTML 5 solutions for video or interactive functions as opposed to Flash embeds. To find out if your site meets the mark, enter your URL on Google’s test site.