This is a symbol of baptism, which now saves you-- not the putting away of the filth of the flesh, but the answer of a good conscience toward God, through the resurrection of Jesus Christ,
What is he talking about? The prison was death by which men were bound. They were held captive. Jesus came to open the prison to those that are bound, or to open up Hades to those people who were bound there, who died before Jesus died for our sins. So when He died, He descended into hell. And He preached to those souls that were in prison. And when He ascended out of hell, He brought with Him those who had been captive.