That sentence always confused me when I heard it in other churches and I never heard a good explanation as to why it was said. Why would Jesus descend into hell? Was he not sinless? Was he not the Son of God? Yes and yes.
The answer comes from tomorrow's Epistle reading from St Peter. Within that text is this sentence: "He was put to death in the flesh, but made alive in the spirit, in which also he went and made a proclamation to the spirits in prison, who in former times did not obey, when God waited patiently in the days of Noah, during the building of the ark, in which a few, that is, eight persons, were saved through water."
It's the "made a proclamation to the spirits in prison" that is the key. Those three days between Good Friday and Easter Morning Jesus descended into hell and proclaimed his salvation to those souls in prison. Logically this makes sense. If the only way to the Father is through Jesus, then those before Jesus would still need his redemptive love.
Isn't that amazing? Even those who had died before him were loved by him. I think St Paul sums it up nicely (Romans 8) that neither life nor death can separate us from the love of God in Christ.
I leave you with an icon of the resurrection. See what Jesus is doing? He's pulling those souls out of prison, one of whom is Adam.