Exodus 18:12 - Ellicott's Commentary On The Whole Bible

Jethro... took a burnt offering and sacrifices for God. — Jethro had brought sacrifices with him, and now offered them in token of his thankfulness for God’s mercies towards himself and towards his kinsman. He occupied a position similar to that of Melchizedek (Genesis 14:18), holding a priesthood of the most primitive character, probably as patriarch of his tribe, its head by right of primogeniture. As Abraham acknowledged rightly the priesthood of Melchizedek (Genesis 14:19; Hebrews 7:2-9), so Moses and Aaron rightly acknowledged that of Jethro. They markedly indicated their acceptance of his priestly character by participation in the sacrificial meal, which, as a matter of course, followed his sacrifice. They “ate bread with Moses’ father in law” (or rather, brother-in-law) “before God.”