Rashi on Leviticus 18:21/רש"י על ויקרא י״ח:כ״א

1למלך [AND THOU SHALT NOT LET ANY OF THY SEED PASS THROUGH THE FIRE] TO MOLECH — This was an idol the name of which was "Molech", and this was the manner in which it was worshipped: that he (the father) handed his child (lit., his son, but it applies to his daughter also; cf. Deuteronomy 18:10) over to the priests of the idol. These lit two large pyres one opposite the other and made the child to pass on foot between the two pyres (Sanhedrin 64b).

2לא תתן refers to “the handing over of the child” to the priests.

3להעביר למלך — this refers to the “passing through the fire".