“Eternal fire” and “fiery Gehenna” are metaphorical presentations of eternal punishment for the sinners. Literal meaning of Gehenna is “the valley of the sons of Hinnom.” Though unknown today, Hinnom must be the name of someone who lived in the ancient Israel.
Gehenna is a deep and narrow valley in the south of Jerusalem. It was famous for idolatrous worship of Molech where people sacrificed children to appease the deity (2 Chr 28:3). The Jewish Law prohibited pagan worship and child sacrifice. However, unfaithful Jews, including King Ahaz (2 Chr 28:1-3), sacrificed their children as burned offering to the false god (Jer 19:4). So, God cursed this valley. King Josiah later stopped the sacrifices here (2 Kgs 23:10).
Later, Gehenna became a place of waste disposal. People burned garbage, including the dead bodies of animals and criminals here. Because of the dumped refuse of the city, it was also a place of worms and maggots. Since the fire kept burning there all the time, this place became a symbol of an everlasting destruction of sinners in the life after death.
Despite all the commandments of God and corrections from prophets, the Jews disobeyed God and practiced idolatry. The influence of evil is strong in the world, and it offers worldly promises to violate the covenant with God. Gehenna is a reminder of the eternal suffering of sinners and salvation of the just in the afterlife.