While Christ's body was in the tomb, his soul was not idle.
Wikipedia's summary of the Catholic view of the Harrowing of Hell is pretty good:
The Catechism of the Catholic Church states: "By the expression 'He descended into Hell', the Apostles' Creed confesses that Jesus did really die and through his death for us conquered death and the devil 'who has the power of death' (Heb 2:14). In his human soul united to his divine person, the dead Christ went down to the realm of the dead. He opened Heaven's gates for the just who had gone before him."As the Catechism says, the word "Hell" — in Latin, infernus, infernum, inferi; in Greek, ᾍδης (Hades); in Hebrew, שאול (Sheol) — is used in Scripture and the Apostles' Creed to refer to the abode of all the dead, whether righteous or evil, unless or until they are admitted to Heaven (CCC 633). This abode of the dead is the "Hell" into which the Creed says Christ descended. His death freed from exclusion from Heaven the just who had gone before him: "It is precisely these holy souls who awaited their Savior in Abraham's bosom whom Christ the Lord delivered when he descended into Hell", the Catechism states (CCC 633), echoing the words of the Roman Catechism, 1,6,3. His death was of no avail to the damned.
Conceptualization of the abode of the dead as a place, though possible and customary, is not obligatory (Church documents, such as catechisms, speak of a "state or place"). When used, this involves assigning to different sections of it the souls of people whether good or evil. In this case it can be said that Christ did not go to the imagined place of the damned, which is what is generally understood today by the word "Hell". For instance, Thomas Aquinas taught that Christ did not descend into the "Hell of the lost" in his essence, but only by the effect of his death, through which "he put them to shame for their unbelief and wickedness: but to them who were detained in Purgatory he gave hope of attaining to glory: while upon the holy Fathers detained in Hell solely on account of original sin, he shed the light of glory everlasting."