What does the Bible mean when it says Christ descended into hell?

It may be that the Lord did descend into the place of confined demonic spirits (tartarus) to make a glorious proclamation of the fact of the victory accomplished through the cross (1 Pet. 3:18-20). But Christ did not go there to redeem anyone nor to release Old Testament saints. As I understand it, God’s redeemed were in Paradise or Abraham’s bosom, probably some place in the heavenlies and when the curtain in the temple was rent, signifying the way into the Holy of Holies had been opened, they were allowed immediate access into God’s presence.

Some try to take the reference in Ephesians 4:8, “When He ascended on high, He led captive a host of captives, And He gave gifts to men.” to refer to the emptying of Paradise or Abraham’s bosom, but such an understanding of this statement is really contrary to its cultural and biblical usage. Behind this is the ancient picture of the Roman triumph where the victorious general in the field of battle road up to the Capitaline Hill on a white horse leading his captives, enemies taken in war, in a triumphant procession. Also in the procession were his troops and the spoils of war which he gave to his soldiers. The Old Testament souls in paradise were not the enemy. This is simply an idiom which means he defeated the enemy and made the enemy his captives. Thus, he was free to give gifts to the church.

The reference to Christ descending to the lower part of the earth is simply a reference to the grave though it may simply be a reference to his incarnation. The following is a good summary of the different opinions on Ephesias 4 taken from The Bible Knowledge Commentary:

Verses 9-11 serve as a commentary on two words of the quotation in verse 8, namely, ascended (vv. 9-10) and “gave” (v. 11). In verses 9-10 Paul commented on the words He ascended. These two verses are parenthetical in thought because the issue of the passage is the giving of gifts. Before Christ could ascend He had to descend. What is meant by to the lower, earthly regions, literally, “into the lower parts of earth”? The genitive “of” can be taken in three ways: (1) “Into the lower parts, namely, the earth” (a genitive of apposition). This would refer to Christ’s incarnation, His “descent” to the earth. (2) “Into the parts lower than the earth” (a genitive of comparison). This would mean that Christ descended into hades between His death and resurrection. (3) “Into the lower parts which belong to the earth” (a genitive of possession). This would refer to Christ’s death and His burial in the grave. The third view best fits the context because in His death Christ had victory over sin and redeemed those who would be given as “gifts” to the church.

In Revelation 6, “Hades” refers to the underworld, the prison and temporary quarters of the souls of unbelievers between their death and the time of the Great White Throne Judgment. This is the compartment called torments in Luke 16:23 and is where all unbelievers are held until the Great White Throne Judgment of Revelation 20. The point of Revelation 6:7-8 is that the aftermath the pale horse rider is war, famine, and death. With war and famine people fall prey to a plague and the wild beasts of the earth and since most are unbelievers, they end up in hades.

Below are definitions of hell and related words:

HELL In common usage, this term refers to the place of future punishment for the wicked. The word properly translated “hell” in the New Testament is the Greek Geenna or Gehenna, a place in the Valley of Hinnom where human sacrifices had been offered and where continuous burning of rubbish made it an apt illustration of the eternal lake of fire (cf. Matt. 5:22). Other words like sheol or hades are improperly translated by this term.

SHEOL The general idea of this word is “the place of the dead” including the grave (cf. Num. 16:30,33; Ps. 16:10), and the unseen place of those who have departed from this life, the place of departed spirits of both the righteous (Gen. 37:35) and the wicked (Prov. 9:18).

HADES This word is basically the New Testament counterpart of the Sheol. It refers to the unseen world in general, but specifically to the abode of the unsaved dead between death and the final judgment at the great white throne (cf. Lk. 16:23 and Rev. 20:11-15). It differs from hell in that it is temporary while hell is permanent.

LAKE OF FIRE Refers to the eternal state of the wicked who are forever separated from God and consigned to a special abode of suffering because of their rejection of Christ or their lack of the righteousness of Christ. It is equivalent to and identified with the second death in Revelation 20:14.