From the summary of the CRS report, Congress’s Contempt Power and the Enforcement of Congressional Subpoenas: Law, History, Practice, and Procedure (May 12, 2017, RL34097):
Congress’s contempt power is the means by which Congress responds to certain acts that in its view obstruct the legislative process. Contempt may be used either to coerce compliance, to punish the contemnor, and/or to remove the obstruction. Although arguably any action that directly obstructs the effort of Congress to exercise its constitutional powers may constitute a contempt, in recent times the contempt power has most often been employed in response to non-compliance with a duly issued congressional subpoena—whether in the form of a refusal to appear before a committee for purposes of providing testimony, or a refusal to produce requested documents.
Congress has three formal methods by which it can combat non-compliance with a duly issued subpoena. Each of these methods invokes the authority of a separate branch of government. First, the long dormant inherent contempt power permits Congress to rely on its own constitutional authority to detain and imprison a contemnor until the individual complies with congressional demands. Second, the criminal contempt statute permits Congress to certify a contempt citation to the executive branch for the criminal prosecution of the contemnor. Finally, Congress may rely on the judicial branch to enforce a congressional subpoena. Under this procedure, Congress may seek a civil judgment from a federal court declaring that the individual in question is legally obligated to comply with the congressional subpoena.
See also, Josh Chafetz, Executive Branch Contempt of Congress, 76 University of Chicago Law Review 1083 (2009). — Joe