He descended into Hell....
Part of the "Apostles' Creed"
When the Apostles' Creed was written (in the 4th Century, we first see it referred to in a letter by St Ambrose in 390 AD) the concept of "Heaven and Hell" as we traditionally understand it did not exist. The original Greek in which the Creed was written uses a word for 'Hell' which translates as 'lowest place'. The Latin into which it was later translated uses a word which means 'those below'. More recent versions use the word 'hell', but it does not mean the 'place of the damned'! In Greek philosophy it was called 'Hades'. In Hebrew, 'Gehenna'. Both ideas refer to a place (or state of being) where all the dead go. There was no distinction back then between Heaven and Hell (and we won't even mention Purgatory!) - these ideas were developed principally in the so-called 'middle ages' sometime between the 5th and 15th centuries.
Very early Christian interpretations of where Christ went after his own death talk of him going to those who had already died since the beginning of the world and leading the 'righteous' to Heaven. The 'damned' receive no benefit from his redeeming death and resurrection.
The idea behind the sentence in the Creed is one of justice. Those who have died before Christ, 'marked with the sign of faith', should not be lost.
More recent Christian thinking on what we used to call "the last things" (theological term for this is 'Eschatology') is taking us away from images such as that in Dante's' Inferno where the souls of the damned are tortured in hell fire for all eternity. The Christian image of God revealed to us by Jesus (as opposed to the OT image of God) is one of infinite love and forgiveness, a Father (Abba) not a vindictive Judge. This does not sit well with eternal torture!
There is a great deal of on-going discussion among theologians about this and there probably will be until Christ returns to sort us all out! The long and the short of it is that we just don't know what happens after death. Christian faith in what Jesus has revealed to us is that he is the 'first born' of a new creation and that the just will all rise again to a new and glorious existence when Christ returns. The jury is still out on what happens to the 'bad guys'. One recent theory from Catholic Theologians is that those who totally reject God during their earthly life will simply 'cease to exist' at the moment of death. A stark idea, but gets over the problem of hell-fire and eternal damnation