While we use the word “hell” today to refer to the place of the damned, where there is fire, and wailing, and gnashing of teeth, the Apostles’ Creed is actually referring to a difference place.
The Greek word that we translate as “hell” in the Creed is hades, which in Biblical times was the name for the abode of the dead, the place deep in the earth where all souls went when they died (sheol is the Hebrew word that refers to the same place). We see from the parable of the rich man and Lazarus (cf. Lk 16:19-31) that, even though all souls went to Hades, their lot was not the same. “Son, remember that you in your lifetime received your good things, and Laz'arus in like manner evil things; but now he is comforted here, and you are in anguish.” For the righteous, Hades was a place of comfort, but for the unrighteous Hades was a place of torment (cf. vs. 22-24).
Note that, while there was some comfort there for the righteous, it still wasn’t heaven, it wasn’t seeing God face to face, with all the knowledge and joy that comes from that vision. After all, the gates of heaven were closed to mankind once Adam and Eve were expelled from the Garden of Eden (cf. Gen 3:22-24). So, Hades was also a place of anxious waiting and longing for the day when the righteous souls could be freed from Hades and enter heaven. All of the patriarchs, and prophets, and holy men and women of the Old Testament were essentially stuck in Hades until someone could come and free them. That someone was Jesus.
When Jesus died and His body was buried, His spirit spent three days in Hades, where “the gospel was preached even to the dead” (cf. 1 Pet 4:6), to “the spirits in prison” (1 Pet 3:19), so that “at the name of Jesus every knee should bow, in heaven and on earth and under the earth” (Phil 2:10). When Jesus descended into Hades, He was not abandoned there (cf. Act 2:27-31). There was no need to worry about who would raise Him from the abyss (cf. Rom 10:6-8). Jesus conquered Hades, and when He rose from the dead, He led with Him a host of captives (cf. Eph 4:8). He holds the keys to death and Hades (cf. Rev 1:17-18), and He has given us the courage to say, "O death, where is thy victory? O Hades, where is thy sting?" (1 Cor 15:55).
That is what we mean when we say that Jesus descended into hell.