Charles Ellicott Commentary Exodus 18:12

"And Jethro, Moses` father-in-law, took a burnt-offering and sacrifices for God: and Aaron came, and all the elders of Israel, to eat bread with Moses` father-in-law before God." — Exodus 18:12 (ASV)

Jethro ... took a burnt offering and sacrifices for God.—Jethro had brought sacrifices with him and now offered them as a 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 Jethro’s priesthood.

They markedly indicated their acceptance of his priestly character by participating 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.