Article 1, Section 5, Clause 3: Congressional Journal

Each House shall keep a Journal of its Proceedings, and from time to time publish the same, excepting such Parts as may in their Judgment require Secrecy; and the Yeas and Nays of the Members of either House on any question shall, at the Desire of one fifth of those Present, be entered on the Journal.

The houses of Congress are required to keep a record of their proceedings. This is not actually the Congressional Record published approximately biweekly when Congress is in session. Although the Congressional Record reports most House debate speeches, it has more of the character of a political magazine and includes a fair amount of material not actually discussed on the floor of Congress, nor in its committees. The official journal is a simple record of actions and votes, published separately, and it is this official journal of each House that satisfies the constitutional requirement.

Not all portions of the official  journal are required to be published, though all actions and votes (including secret ones) are included in the official journal. Voice votes, for which no record is made of how specific members voted, are typically on routine procedural matters. However, the demand of one-fifth of the members is enough to force a roll call vote that is recorded in the journal. This is most likely used primarily as a delaying tactic. The existence of this motion to take a roll call vote is nonetheless important, as it establishes the rule that votes of Congress are not to be by secret ballot, but must be made openly. This prevents members of Congress from making secret deals adverse to the interests of the district or state they represent.