The way it was explained to me, the word “hell” is, today, used to mean exclusively, the place of the damned. But in antiquity, and this is an ancient prayer to which you are referring, the word “hell” had a wider meaning. It used to mean, the place where all the souls, good and evil, were gathered when they died.
If you look up other versions of the Apostles Creed, you’ll see that some say, “He descended to the dead”.
Anyway, Jesus descended to that place in order to gather the righteous souls of the Old Testament Hebrews. This is how Scripture describes the situation:
1 Pet 3: 18 Because Christ also died once for our sins, the just for the unjust: that he might offer us to God, being put to death indeed in the flesh, but enlivened in the spirit,
19 In which also coming he preached to those spirits that were in prison:
Hebrews 9:15 And therefore he is the mediator of the new testament: that by means of his death, for the redemption of those transgressions, which were under the former testament, they that are called may receive the promise of eternal inheritance.
See also,
Hebrews 11, to see the names of many of the Hebrews who were led out of the place of the dead, by Jesus.