Five Possible Views:
View 1: When Noah was building the ark, Christ ‘in spirit’ was in Noah preaching repentance and righteousness through him to unbelievers who were on the earth then but are now ‘spirits in prison’ (people in hell).
View 2: After Christ died, he went and preached to people in hell, offering them a second chance of salvation.
View 3: After Christ died, he went and preached to people in hell, proclaiming to them that he had triumphed over them and their condemnation was final.
View 4: After Christ died, he proclaimed release to people who had repented just before they died in the flood, and led them out of their imprisonment (in Purgatory) into heaven.