Google Mobile Friendly Website Tester
In April 2015 Google made the largest change ever to their algorithm of how they rank websites in the search engine rankings and it was all about being mobile friendly. Basically if your site is mobile friendly, you have a shot at a decent ranking or SEO, but if you aren’t mobile friendly you have almost no chance of succeeding. You can test this yourself at http://google.com/webmasters/tools/mobile-friendly/