A set or series of written questions drawn up for the purpose of being propounded to a party, witness, or other person having information of interest in the case. They are a pretrial discovery device consisting of written questions about the case submitted by one party to the other party or witness. The answers to the interrogatories are usually given under oath.

In Plain English:

Interrogatories are a tool used during the discovery process. They are questions that attorneys write that are given to the other side and their witnesses to answer. This helps the attorneys learn information that is necessary to understand what happened in the case.