Google possesses the authority to issue manual actions against a website that is found to be in violation of its Webmaster Quality Guidelines. This action can lead to a website being penalized in the search results or even being completely removed from the index.
While Googlebot can easily detect generalized spam and take automated action against suspicious sites during the indexing process, manual actions provide an added layer of security because they are carried out by human reviewers instead of bots. These manual actions serve to further prevent individuals from manipulating search rankings through the use of black hat SEO practices.
If your site is flagged with any manual actions, individual pages or the entire website may experience a significant drop in the search results pages. There are several different types of manual actions, including those for pure spam, thin content, structured data issues, unnatural links, and keyword stuffing.
You can check to see if your site has any manual actions against it by consulting the Manual Actions Report within Google Search Console. Here, you can also formally request that Google review your site (after you have corrected the underlying problem) and remove any active manual actions.