A big Google penalty where a human reviewer concluded that your website did not follow Google’s webmaster quality guidelines. A site that receives a manual action either gets their rank greatly reduced or they get removed from the SERPs completely. You should then try to repair the issues and apply for reconsideration when everything is fixed.
You can check if your site has a manual action (and which pages are affected) here:
https://search.google.com/search-console/manual-actions
