Albert Barnes Commentary Joshua 15:8

"and the border went up by the valley of the son of Hinnom unto the side of the Jebusite southward (the same is Jerusalem); and the border went up to the top of the mountain that lieth before the valley of Hinnom westward, which is at the uttermost part of the vale of Rephaim northward;" — Joshua 15:8 (ASV)

The valley of the son of Hinnom - This valley begins on the west of Jerusalem at the road to Joppa, and turning southeastward around the foot of Mount Zion joins the deeper valley of Kedron on the south of the city. It was in this ravine, more particularly at Tophet in the more wild and precipitous part of it toward the east, that the later kings of Judah offered the sacrifices of children to Moloch (2 Chronicles 28:3; 2 Chronicles 33:6, etc.).

After these places had been defiled by Josiah, Tophet and the whole valley of Hinnom were held in abomination by the Jews, and the name of the latter was used to denote the place of eternal torment (Matthew 5:22). The Greek term Gehenna (γεέννα geenna) is in fact formed from the Hebrew הנם גיא gay' hı̂nnôm, “valley of Hinnom.” Hinnom is regarded either as the name of some ancient hero, or as an appellative (“groaning” or “moaning”), bestowed on the spot because of the cries of the victims offered here to Moloch, and of the drums with which those cries were drowned.

The valley of the giants - Rather “the plain of Rephaim.” This plain, named after an ancient and gigantic tribe of the land (Genesis 14:5), lies southwestward of Jerusalem, and is terminated by a slight rocky ridge forming the brow of the valley of Hinnom. The valley is fertile (Isaiah 17:5) and broad, and has been on more than one occasion the camping ground for armies operating against Jerusalem (2 Samuel 5:18; 2 Samuel 5:22; 2 Samuel 23:13).