An interrogatory is similar to a motion for discovery.
In a discovery motion, one party asks the court to order another to provide specific information related to a case. This is usually done when a party has been less-than-forthcoming with requested documents.
If the judge grants the discovery motion and the party still does not comply, they can be held in contempt of court.
Once the information sought is provided, the requesting party examines it in preparation for a trial or to decide whether to file further complaints.