The Literal Commandment
You shall not give any of your offspring to offer them to Molech, nor shall you profane the name of your God; I am the LORD. (Leviticus 18:21)
The literal commandment is that we should not give any of our offspring (children) to offer them to Molech.
Molech (also called Moloch) was a god of the ancient Phoenicians. Worship of Molech usually involved passing children through a fire built in the lap of a Molech idol. In some rabbinic accounts, the idol was hollow and was divided into seven compartments, in one of which they put flour, in the second turtle-doves, in the third a ewe, in the fourth a ram, in the fifth a calf, in the sixth an ox, and in the seventh a child, which were all burned together by heating the statue inside.1
In mild cases the child was simply passed through the fire with little to no injury. In extreme cases the child was completely burned alive as a sacrifice to Molech.
This Leviticus passage specifically mentions Molech while a related passage in Deuteronomy 18:10 simply prohibits "passing [one's] son or daughter through the fire". 2 Kings 16:3 and 2 Kings 23:10 both mention this.