The Valley of Hinnom had a very horrendous history in ancient times. It was used as a place where the pagan worshipers did all sorts of vile and wicked things - including burning children alive as sacrifices to the idols Moloch and Baal. One section of the valley was called Tophet, or the "fire-stove," where the children were slaughtered (2 Kings 23:10). It was a place of tremendous evil for many years.
After their return from the Babylonian exile (see Why Babylon?), the Jews turned the Hinnom Valley into the city dump where garbage and anything deemed unclean (including the bodies of executed criminals) was incinerated. For that purpose, a fire was kept constantly burning there. Even though it was no longer used for evil worship, with all the filth and thick smoke it remained a very dark and dreary place.
The Hebrew name Hinnom when translated into Greek is gehenna, from which the word and concept of hell originated. By the time of Jesus Christ, the deep, constantly-burning Valley of Hinnom was also known as the Valley of Gehenna, or Hell, and had taken on a popular image as the place "down there" where the wicked would eventually be cast into the flames for destruction.
Fact Finder: Did Jesus Christ also refer to the fire of Gehenna?
Note: Jesus used the original word Gehenna, which is translated hell, in the above Scripture, but He obviously wasn't referring to the fires of the Valley of Hinnom - He was talking about the future lake of fire that will be used to destroy the enemies of God (Revelation 19:20, 20:13-15).