A Vaughn index is a tool used in public records disputes where the person requesting documents, will ask the court to compel the records custodian to list all responsive documents and explain why or why not the documents should be released.
Okay, this is complex topic. Let me try to explain this concept in another way. If there is a denial of a public records request, then generally one needs to file some sort of motion to compel the production of the requested documents with the appropriate court. But the records custodian has all the documents? The problem is both the court and the requester does not know if the records custodian considered all documents. Thus, a Vaughn index works as a secondary motion to compel, where the court tells the records custodian to make a list of all responsive documents and to legally justify their conclusion.