As a place of sacrifice
The name Gehinnom was originally attached to a literal place outside Jerusalem. Prior to the reign of Josiah, the valley was allegedly the site of child sacrifices by fire, comparable to those unearthed at Carthage. During his reign, the temple at which these sacrifices took place was destroyed.
As a punishment in the afterlife
In Rabbinic Judaism, the name Gehinnom is used for a purgatory-like place of atonement for sins. Souls enter it after their death, and spend upwards of a year there. Afterwards, they either ascend to olam ha-ba, the world to come, if righteous, or turn to ashes if not. The exceptions are really bad sinners, who just stay there forever.
In Christian texts, the term Gehenna is used to reference boring old Hell.