On April 21, 2015 Google went public with a change to their algorithm that sent website users into a panic. As of this date, it was reported mobile-friendly websites would rank better than non-mobile optimized websites.
According to MOZ a leading source of Google algorithm information “In a rare move, Google pre-announced an algorithm update, telling us that mobile rankings would differ for mobile-friendly sites starting on April 21st. ”
Mobile-first indexing means Google predominantly uses the mobile version of the content for indexing and ranking. Since the majority of users now access Google Search with a mobile device, Googlebot primarily crawls and indexes pages with the smartphone agent going forward. If your website is one of the original looks or the “Kuni 920 widescreen” template (see below for examples) your search rankings are being negatively affected. The solution is to update to a mobile-ready responsive template.