Ephesians 4
(Now this, “He ascended”—what does it mean but that He also [d]first descended into the lower parts of the earth? 10 He who descended is also the One who ascended far above all the heavens, that He might fill all things.)

Commentary:
Some think that the phrase lower parts of the earth refers to Jesus’ preaching to the spirits in prison described in 1 Peter 3:19 and 4:6. While this aspect of Jesus’ ministry in Hades following His work on the cross is true (and prophesied in Isaiah 61:1-2 and Luke 4:18), Paul did not necessarily refer to it here.

I do not understand what it means to preach to the “spirits in prison.”

any thoughts?

Luke 4
18 “The Spirit of the Lord is upon Me,
Because He has anointed Me
To preach the gospel to the poor;
He has sent Me [i]to heal the brokenhearted,
To proclaim liberty to the captives
And recovery of sight to the blind,
To set at liberty those who are [j]oppressed;