The spirits in prison
'He went and preached to the spirits in prison' (also 1 Peter 4:6) — the descent to the dead and harrowing of hell, preaching through Noah, or the imprisoned angels; the creed's 'descended' reads here.
1 Peter 3:18-22
18Because Christ also suffered for sins once, the righteous for the unrighteous, that he might bring you to God, being put to death in the flesh, but made alive in the Spirit,
19in whom he also went and preached to the spirits in prison,
20who before were disobedient, when God waited patiently in the days of Noah, while the ship was being built. In it, few, that is, eight souls, were saved through water.
21This 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,
22who is at the right hand of God, having gone into heaven, angels and authorities and powers being made subject to him.
What each tradition says
Rabbinic / Midrashic
Emil G. Hirsch’s entry says that in the Zohar Aza and Azael fall and are punished by being chained to the mountains of darkness. Another passage says these two rebelled against God and were hurled from heaven, and now teach men all kinds of sorcery.
Emil G. Hirsch (1851-1923), The Jewish Encyclopedia, vol. 5, s.v. 'Fall of Angels' (New York: Funk & Wagnalls, 1901-1906) — section 'Later Jewish Tradition'
Patristic
Irenaeus states that the Lord descended into the regions beneath the earth and preached His advent there. He also declared the remission of sins received by those who believe in Him.
Catholic
Thomas Aquinas says a thing is in a place first through its effect. By descending, Christ affected each hell differently: he put the lost to shame for their unbelief and wickedness, gave those in Purgatory hope of glory, and shed everlasting glory's light on the holy Fathers detained only for original sin.
Thomas Aquinas (c. 1225-1274), Summa Theologica, trans. the Fathers of the English Dominican Province (1920; newadvent.org text) — III, q. 52, a. 2 (Whether Christ went down into the hell of the lost?), corpus of the reply
Orthodox
John of Damascus writes that after He had freed those who had been bound for ages, He straightway rose again from the dead. He thereby shows us the way of resurrection.
John of Damascus (c. 675-749), An Exact Exposition of the Orthodox Faith (trans. S. D. F. Salmond), in Nicene and Post-Nicene Fathers, ser. 2, vol. 9 (Edinburgh: T&T Clark, 1898) — Book III, ch. 29 ('Concerning the Descent to Hades')
Reformed / Protestant
Calvin has no doubt that Peter speaks generally: Christ’s grace was manifested to godly spirits, who were endued with the Spirit’s vital power. Therefore, there is no reason to fear it will not flow to us.
John Calvin (1509-1564), Commentaries on the Catholic Epistles (trans. John Owen; Edinburgh: Calvin Translation Society, 1855) — Commentary on 1 Peter 3:19
Evangelical
Wesley says that by the Spirit he preached through Noah’s ministry. The spirits in prison are the unholy men before the flood, reserved by God’s justice as in a prison until he executed the sentence on them all. They are now also reserved to the judgment of the great day.
John Wesley (1703-1791), Explanatory Notes upon the New Testament (1755; CCEL's 'Wesley's Notes on the Bible') — Note on 1 Peter 3:19