What does this phrase of the CREED mean: “Jesus descended into hell”?
Scripture calls the abode of the dead, to which the dead Christ went down, "hell" - Sheol or Hades- because those who are there are deprived of the vision of God. "It is precisely these holy souls, who awaited their Savior in Abraham's bosom, whom Christ the Lord delivered when he descended into hell. "Jesus did not descend into hell to deliver the damned, nor to destroy the hell of damnation, but to free the just who had gone before him. (Catechism of the Catholic Church #633).
Saint Peter also teaches: "The gospel was preached even to the dead."(1Pe. 4:6) The descent into hell brings the Gospel message of salvation to complete fulfillment. This is the last phase of Jesus' messianic mission, a phase which is condensed in time but vast in its real significance: the spread of Christ's redemptive work to all men of all times and all places, for all who are saved have been made sharers in the redemption.
(Catechism of the Catholic Church #634).