Gehenna is a geographical place - a ravine just outside the old city of Jerusalem. It was once a place of human sacrifice - including infant sacrifice. Later it became a refuse dump, with a perpetual fire burning the rubbish, garbage and corpses thrown there.
For Jews, this place became an image of the terrible state persons would be in if they chose to separate themselves from God and, as a result, remained in that state for all eternity.
The Book of Isaiah ends with this image. The prophet pictures the end of time when the just will come to the new Jerusalem and as they draw near to the city, they will see this ravine with God's enemies in it:
On their way they will see the corpses of those
who rebelled against me; for their worm will
never die, nor their fire be put out. And they
will be held in horror by all people.