Manual Action Penalties are punitive actions that a human employee at Google will make if they spot that a website is violating the search engine’s quality guidelines. This is a penalty that is instated by people rather than Google’s algorithms and crawler bots.
The penalty can range in severity. It might demote a website’s rank or remove that webpage or website from their index altogether. Usually, the severity of the penalty will correspond with the seriousness of the breach made to Google’s guidelines.
If your website has been the subject of Manual Action Penalties, you can find out via Google’s Search Console.