What do we mean when we say that Christ conquered death?

Before the Incarnation of Christ and His Resurrection, souls of the dead were in a condition of rejection, separated from God, in darkness, hell, Sheol. In hell were also the souls of the Old Testament righteous ones. Our deliverance was also their deliverance. Christ after His death descended in His soul and divinity into hell, while His body remained in the grave. He preached salvation to the captives of hell and brought up from hell all the Old testament righteous ones into the bright mansions of the Kingdom of Heaven.

For Christ also suffered once for sins, the just for the unjust, that He might bring us to God, being put to death in the flesh but made alive by the Spirit, by whom also He went and preached to the spirits in prison. (I Peter 3:18-19)

For this reason the gospel was preached also to those who are dead, that they might be judged according to men in the flesh, but live according to God in the spirit. (I Peter 4:6)

Now that He ascended, what does it mean but that He also first descended into the lower parts of the earth? He who descended is also the One who ascended far above all the heavens, that He might fill all things. (Eph 4:8-10)

As a result, death for righteous men became only a transition from the world below to the world above, to a better life, to life in the Light of the Kingdom of God; bodily death itself became only a temporary phenomenon, for by the Resurrection of Christ the way to the General Resurrection was opened to us.