Sites have different ranking on Google when using a mobile device compared to a desktop computer. During this update, major changes were observed in Google's mobile version only (not on Google results on desktop). These changes were reported by different tools measuring mobile SERP and also in communities like Webmaster World and Black Hat World. Some of these reports described it as "Google dance" a term that describing acute movements in results for several hours or days (before “getting back to normal”) and is rarely used today.
Mobile First Index is another update that was announced before this update. According to Google's statement, starting from Mobile First Index update, Google will crawl the mobile version of each site (before crawling the Desktop version) and will use the Mobile version to determine site rankings.
Initially, people assumed that this update was an update that preceded Google's official move to Mobile First Index, but then this claim seemed less true because the actual Mobile First Index update did not cause any significant ranking changes as opposed to this update's Significant changes.
What can be done?
In case you were affected by this update, you probably received (at some point) a message from Google informing you that your site is not completely compatible with mobile. If you have not received such a message or you still haven’t connected to Google Webmaster Tools (Search Console), do so as soon as possible and check your status through the Search Console report.