Mobilegeddon was a Google algorithm update that started to be rolled out on April 21st, 2015. This update looked at a website and determined, either yes or no, if the site was deemed to be “mobile-friendly”. There was (and still isn’t) a kinda score. You either are mobile-friendly or you’re not.
This update was designed to give websites that are mobile-friendly a boost of their search engine ranking signal on Google’s mobile search results page. So, in other words, when someone searches for your site on a mobile device your rankings could see a boost on those mobile search results pages.
But, many people never “saw” a major impact after Mobilegeddon. As this update rolled out, the results had been more gradual than the pundits had predicted so there was no brink of disaster. That doesn’t mean that change wasn’t happening. It was and the warnings may have been too ominous!