A "Google Update" refers to update to Google's way of ranking websites in the organic search results.
Usually, this means a change to many, if not thousands of websites positively and negatively.
The reason why such a "shake-up" happens is that Google develops new filters and signals "on top" of the core algorithm first. Such updates have been happening since the inception of the Google search engine. In the early 2000s, these updates were also sometimes referred to as "Google Dance," which gave a popular webmaster relations event at the Google Plex its name as well.
The "Google Pagerank Updates" were very popular and anxiously awaited for over a decade because SEOs were expecting a new "public toolbar PageRank" value. Only very few marketers knew how to access the "real rank" or "internal page rank" and took advantage of this "time machine for page rank."
The very well-known Google Penguin Update has shaken up the online marketing industry for years 2012 until 2016,
until it was integrated into the Google core algorithm like so many other "Google Quality Signal Updates."