Jesus descended "ad inferos", as the Apostle's Creed puts it. That is "to the lower regions" or "netherworld" i.e. to the place of the dead: She'ol/Hades.
We usually translate this into English as "descended into hell", because "hell" is a word that tends to be a catchall for several biblical concepts.
Hell is used to refer to She'ol/Hades, i.e. the place of the dead.
Hell is used to refer to Gehenna, the experience of suffering which the wicked dead undergo.
And we often use "Hell" to describe the lake of fire in St. John's Apocalypse.
So "Hell" does a lot of heavy lifting, and requires unpacking.
In the 2nd Temple Period of Judaism there was this idea that the righteous would experience a foretaste of future glory (the resurrection and Olam Ha'ba or Age to Come); this was often imagined as the Garden of Eden (Gan-Eden), that's what "Paradise" refers to. Our word "paradise" is taken from the Greek paradeisos, itself a borrowing of a Persian word paradaeza, "an enclosed garden". The Persian word literally referred to a walled off garden, or what we'd call a park. This word was borrowed into Greek and in a Jewish context was used to mean the Garden of Eden. In 2nd Temple period literature there was an idea that since the Garden of Eden had been lost and closed off from humanity after Adam and Eve's expulsion, that it was no longer here on earth, but had been taken up by God. In apocalyptic literature of the time, such as the apocalypse of Enoch the Garden of Eden is described as existing in "the third heaven". We have to remember that in the ancient near east there were numerous "heavens" referring to the skies, that which is above the earth. The standard was seven heavens with the seventh being the greatest and highest "the heavens of heavens" as the divine throne room--hence Solomon says "The heavens cannot contain You, not even the heavens of heavens". Though in the Enochian texts, perhaps through Hellenistic influence, describes ten rather than seven. Regardless, the idea is that the Garden of Eden or Paradise exists in the heavens, beyond mortal reach, but is a place of rest for the righteous. This is also why St. Paul locates Paradise in the third heaven.
Yet Jesus also describes the place of rest for the righteous as "Abraham's Bosom", that is, the righteous are with Abraham (and all the Patriarchs). And in Jesus' parable of Lazarus and Dives He describes the place of the righteous dead and the place of the wicked dead as separated by an impossible to cross gulf. This idea therefore grounds the place of the righteous dead as a kind of "portion" of She'ol--the universal grave.
When we confess that Jesus descended "into hell" we are not talking about Jesus "going to the place where the wicked suffer", it's about Jesus entering the realm of death and the dead. And it is to proclaim victory against the cosmic powers of darkness, i.e. death and the devil who wields the power of death. St. Peter describes the Lord as going to the prison where the spirits of the dead who fell were, or to where the fallen angels were kept in chains--Jesus preaches--proclaims His victory over all the cosmic powers of darkness. His death is not His defeat, but the means by which He actually destroys and defeats the power of death--that by His resurrection He lifts up that which has been held in the bondage of death (human beings specifically, but ultimately all of creation) thereby defeating and destroying that power and signaling the ultimate doom for the ancient adversary of God--the devil.
So Christ's descent is an invasion--Christ invades death robbing death of its power, unshackling the chains, freeing the captives, setting liberty to the prisoners i.e. to all men.
That is the descent into "hell"; that is why Hell is harrowed--conquered, defeated, and robbed of its vice-grip of death over creation.
Christ is risen from the dead
Trampling down death by death
And to those in the tombs
Bestowing life
-CryptoLutheran