Form I-824, Application for Action on an Approved Application or Petition, is used to file for a duplicate approval notice (also known as Form I-797, Notice of Action).
Form I-797, Notice of Action, is the document issued when an application or petition to the USCIS has been approved.
The Application for Action on an Approved Application or Petition is used by immigrants who need an additional copy of their notice of action for immigration purposes or otherwise.