Mobilegeddon, as it came to be known, was a major Google SEO algorithm change that occurred in April 2015. Essentially, what it meant for SEO was preference was given to sites or content that was mobile-friendly. Initially, it applied only to searches on mobile devices and smartphones.
The announcement sent the world of SEO into a whirl. Although it did not affect normal searches on a PC, given the large and growing number of mobile searches, it was still a significant change. More than half of all Google searches are conducted on a mobile device, and this number is growing steadily.