What is a Manual Action?
Google can issue manual actions against a site that violates its Webmaster Quality Guidelines. This action may result in a website being penalized in the search results or completely removed.
More About Manual Actions
However, manual actions provide an added layer of security because they are carried out by humans instead of bots. These manual actions further prevent people from manipulating search rankings via black hat SEO practices.
If your site has any manual actions against it, individual pages or the entire website may drop significantly in the search results pages. There are different manual action types: 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 opening the Manual Actions Report in Google Search Console. Here, you can also request that Google review your site (after fixing the problem) and remove any manual actions.