Several times a year google will make some significant changes to the search algorithms and systems. These are known as Core Updates. These are listed on the List of google Search Ranking Updates
Usually when google makes a core update, most websites don’t need to worry about it, and sometimes dont even notice that an update has occurred. Usually if you notice a significant change in traffic that correlates with Google Updates, then you may need to assess or improve your site.
“Core updates are designed to ensure that overall, we’re delivering on our mission to present helpful and reliable results for searchers. These changes are broad in nature, and don’t target specific sites or individual web pages. As content on the web changes, we assess and update our systems to keep pace, as a whole.”