A more thorough explanation:

A Vaughn index is a list of all the documents that the government wants to keep secret in a Freedom of Information Act (FOIA) lawsuit. Each document on the list is accompanied by a statement explaining why the government wants to keep it secret. The Vaughn index is supported by one or more affidavits.

The Vaughn index serves three purposes:

1. It forces the government to carefully consider all the material it wants to keep secret.
2. It helps the court decide whether the government's reasons for keeping the material secret are valid.
3. It gives the person who requested the information as much information as possible about why the government wants to keep certain documents secret.

The name "Vaughn index" comes from a court case called Vaughn v. Reese, which was decided in 1973 by the United States Court of Appeals for the District of Columbia Circuit.

Imagine that a journalist files a FOIA request for all the emails between a government official and a lobbyist. The government responds by providing some of the emails, but withholds others, claiming that they contain confidential information that should not be made public. The government must then create a Vaughn index listing all the emails it is withholding and explaining why it wants to keep them secret. The journalist can then challenge the government's decision to withhold certain emails by arguing that the government's reasons are not valid.