The phrase, "he descended into hell" comes from the Apostles Creed.
632 The frequent New Testament affirmations that Jesus was "raised from the dead" presuppose that the crucified one sojourned in the realm of the dead prior to his resurrection. This was the first meaning given in the apostolic preaching to Christ's descent into hell: that Jesus, like all men, experienced death and in his soul joined the others in the realm of the dead. But he descended there as Savior, proclaiming the Good News to the spirits imprisoned there.
633 Scripture calls the abode of the dead, to which the dead Christ went down, "hell" - Sheol in Hebrew or Hades in Greek - because those who are there are deprived of the vision of God. Such is the case for all the dead, whether evil or righteous, while they await the Redeemer: which does not mean that their lot is identical, as Jesus shows through the parable of the poor man Lazarus who was received into "Abraham's bosom": "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.
634 "The gospel was preached even to the dead." 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.
635 Christ went down into the depths of death so that "the dead will hear the voice of the Son of God, and those who hear will live." Jesus, "the Author of life", by dying destroyed "him who has the power of death, that is, the devil, and [delivered] all those who through fear of death were subject to lifelong bondage." Henceforth the risen Christ holds "the keys of Death and Hades", so that "at the name of Jesus every knee should bow, in heaven and on earth and under the earth."
Today a great silence reigns on earth, a great silence and a great stillness. A great silence because the King is asleep. The earth trembled and is still because God has fallen asleep in the flesh and he has raised up all who have slept ever since the world began. . . He has gone to search for Adam, our first father, as for a lost sheep. Greatly desiring to visit those who live in darkness and in the shadow of death, he has gone to free from sorrow Adam in his bonds and Eve, captive with him - He who is both their God and the son of Eve. . . "I am your God, who for your sake have become your son. . . I order you, O sleeper, to awake. I did not create you to be a prisoner in hell. Rise from the dead, for I am the life of the dead."
The bottomline answer is that this place is NOT the place of the damned that we usually call "hell". Hell or Hades had two sides. One side is where the saved (saints) of the Old Testament waiting for the coming of Jesus to lead them out of Hades and into heaven. The other side of hell (Hades) is where the damned resided. This side of Hades is what we today call "hell".
Jesus descended into the place of the dead (Hades) called Abraham's bosom where the saints of the Old Testament waited for him. Jesus preached the Gospel to them and lead them out of hades and into heaven.