The "Harrowing of Hell" is the ancient church teaching that when Jesus died he "descended into hell" as the Apostles and Nicene Creed teaches. Well it says he went to Hades which could just mean he died. He was dead. And when the creeds were developed they were fighting some notions that he didn't really died, just looked like he died. The Creeds say, no he died and he went where people who die go. Wherever that is.
But there is that interesting line from I Peter 3:19
He was put to death in the flesh, but made alive in the spirit, in which also he went and made a proclamation to the spirits in prison, 19
Biblical scholars really don't know what to make of that. But the early church, particularly the Eastern church knew what to do with that. Jesus stormed the gates of hell and freed Adam and Eve and all those imprisoned by sin, death and the devil.
The Acts of Pilate, a third century apocryphal text describes the Harrowing of hell in this way:
The bronze gates were broken in pieces and the bars of iron were snapped; and all the dead who were bound were loosed from their chains, and we with them. And the King of Glory entered like a man, and all the dark places of Hades were illumined..
I'm very taken with this image. This is what I said last year about that:
.....there is something very appealing about the ancient traditions of Jesus invading hell itself and releasing all the souls trapped there. If Hell is the place of total darkness and absence from God, Jesus' invasion of it shows there is no place that can hold God back. There is no darkness Christ's Light cannot illumine. In fact, if Hell is the absence of God, and Christ has invaded Hell, it no longer even exists! There is no longer exists a place where God is absent.
There is a beautiful homily on Jesus freeing Adam from hell on Holy Saturday by an anonymous Eastern priest from maybe third century that I posted here last year.