A more thorough explanation:

Definition: Frank refers to the authority given to individuals or officials to send official mail related to their position without charge. This privilege has been around for many centuries in English commonwealth nations. In the United States, franking privileges mostly refer to members of Congress who have had franking privileges off and on since the first session of Congress in 1789.

For example, members of Congress needed franking to be able to communicate with constituents, but after some members abused the privilege to gain election advantages, the House and Senate each limited the mailing privileges to “official business” and limited the overall amount of mass mailing. Presently, members can spend only as much on mailing as allowed by an “official mail allowance” set for each office to account for the amount of constituents served. Other franking privileges have been given to officials such as former presidents and the vice president. Sometimes during war time, franking privileges have been bestowed upon soldiers to communicate with family members.

Frank also refers to a hearing or motion in a criminal case used to challenge a police officer having probable cause to conduct a search and seizure. If the motion is successful, any evidence acquired during the illegal search will be struck from the record.

For example, if a police officer conducts a search without probable cause and finds evidence of a crime, a defense attorney may file a Frank motion to challenge the legality of the search. If the motion is granted, the evidence found during the search cannot be used in court.

In summary, Frank refers to the privilege of sending official mail without charge and a legal motion used to challenge the legality of a search and seizure.