1. An orthodox Protestant explanation
Grudem concludes : Our conclusions on this passage may now be expressed in an extended paraphrase: “In the spiritual realm of existence Christ went and preached through Noah to those who are now spirits in the prison of hell. This happened when the patience of God was waiting in the days of Noah while the ark was being built”[Wayne Grudem, 1 Peter, Tyndale Commentary 1988; Appendix p.239].
2. An explanation without requiring immortal souls
“to open the eyes that are blind, to bring out the prisoners from the dungeon, from the prison those who sit in darkness. (Isaiah 42:7)
“But this is a people plundered and looted; they are all of them trapped in holes and hidden in prisons; they have become plunder with none to rescue, spoil with none to say, “Restore!” (Isaiah 42:22)
saying to the prisoners, ‘Come out,’ to those who are in darkness, ‘Appear.’ They shall feed along the ways; on all bare heights shall be their pasture; (Isaiah 49:9)
The Spirit of the Lord God is upon me, because the Lord has anointed me to bring good news to the poor; he has sent me to bind up the brokenhearted, to proclaim liberty to the captives, and the opening of the prison to those who are bound; (Isaiah 61:1)