During the pre-trial "discovery" process in civil cases, the parties trade information about the issues, witnesses, and other evidence in the case. This process prevents a "suprise" at trial and allows the opposing party to be ready with its own evidence. "Interrogatories," one method of discovery, are written questions directed to an opposing party to be answered under oath and in writing.
American Bar Association, Discovery, How Courts Work (Sept. 9, 2019), https://www.americanbar.org/groups/public_education
/resources/law_related_education_network/how_courts_work/discovery/.