Molech was an Avodah Zara that was worshipped by giving one’s child to the priest who would then lead the child through 2 bonfires. The Torah is telling us that one should not give over his child to the priests for such worship.
The Posuk places the prohibition of having relations with an Aishes Ish (a married woman) next to worshipping the Avodah Zara Molech. What is the connection?
Rabaynu Bachyeh answers, when one’s wife has an illicit relationship with another man, this brings about a jealousy in her husband. So too, when one worships the Molech, this brings about a jealousy in Hashem (Kaviyachol). Although it is not really within our comprehension to know what saying “Hashem is jealous” means, we do find this in other places concerning Avodah Zara. In Parshas Yisro, by the Aseres HaDibros, the posuk warns not to worship Avodah Zara, and there too it says that Hashem will not forgive because he is “Kail Kana”- a jealous G-d.