What does "He descended into hell" mean?
When we say that Jesus descended into hell we mean that after His death on the cross, while His body remained in the tomb, Jesus' soul descended "into the lower parts of the earth." Since the gates of heaven were closed with Adam's original sin, and could not be reopened until the Redemption, the patriarchs and other just of the Old Testament (i.e., all people since Adam) who were not condemned to the hell of eternal punishment were sent to the so-called vestibule of hell, what Catholic Tradition calls the Limbo of the Fathers.
The doctrine of Christ's descent to the dead, which was defined by the Fourth Lateran Council as divinely revealed, is explicitly taught in the Bible.
"But God raised him up, having loosed the sorrows of hell, because it was not possible that he should be held fast by it" (Acts 2:24; cf. 31).
"Now this, he ascended, what does it mean but that he also first descended into the lower parts of the earth?" (Ephesians 4:9)
"In which also he went and preached to those spirits that were in prison" (I Peter 3:19).
In the New Testament, Christ Himself refers by various names and figures to the place or state which Catholic tradition has agreed to call the Limbo of the Fathers. In Matthew 8:11 it is spoken of under the figure of a banquet, or in Matthew 25:10 under the figure of a marriage feast. He also calls it "Abraham's bosom" in the parable of Lazarus and Dives (Luke 16:22), and "paradise" when addressing the penitent thief on the cross (Luke 23:43).