Tophet was established of old, yes, for the king it is prepared. He has made it deep and large; its pyre is fire with much wood; the breath of the Lord, like a stream of brimstone, Kindles it. (Isaiah 30:33)
Tophet is metaphorically used to somewhat describe hell. It was a literal place in the valley of Hinnom where children were sacrificed by fire to the heathen god Moloch. It was an awful place, a place of torment and suffering. Thus it is appropriately used to describe the place were those refusing redemption will spend eternity. As Tophet was a real place in the valley of the sons of Hinnom it is likewise a real place in eternity. It is the place where the justice of God sentences those without the saving knowledge of His Son. It is the place where those who have rejected God's mercy are condemned. It is such a just place that, "The breath of the Lord, like a stream of brimstone, kindles it." We may not like to hear it because it goes against man's misguided view of tolerance. Nevertheless God in His justice must merit out judgment to those who have rejected His mercy.