Exodus 18:12

And Jethro took burnt offerings and holy sacrifices before the Lord, and Aharon and all the elders of Israel came to eat bread with the father-in-law of Mosheh before the Lord; and Mosheh stood and ministered before them.

AND JETHRO, MOSES’ FATHER-IN-LAW, TOOK A BURNT-OFFERING AND SACRIFICES FOR G-D. All this took place before they came to Mount Sinai., where Ramban develops at length his explanation that Jethro, having arrived at the mount of G-d, sent a messenger to Moses in Rephidim, informing him of his arrival. Moses thereupon went out to meet him and brought him to Rephidim, where this feast took place. It was thus before Israel’s arrival at Mount Sinai that all this took place. It is also possible to explain that Scripture arranged the entire narrative of Jethro [in one section] even though this particular event occurred after he had stayed with the Israelites a long time and, in the meantime, became converted through circumcision, immersion, and the sprinkling of the blood of a sacrifice, according to the law., i.e., of the Passover-offering (above, 12:48). Immersion is mentioned further (19:10), and so is the bringing of a sacrifice (24:5). At the time of the Sanctuary, immersion and a sacrifice were required of a female proselyte. Nowadays, immersion alone is the prerequisite to her entrance into the covenant. See Rambam, Mishneh Torah, Hilchoth Isueri Biah, Chapter 13, for further eludication of these principles. See also Ramban further, 19:10.

AND AARON CAME, AND ALL THE ELDERS OF ISRAEL, TO EAT BREAD. That is, with Jethro in the day of his espousals, [i.e., in the day of his entrance into the covenant with G-d], for he was then as a newly-circumcised child.

ויבא אהרן וכל זקני ישראל לאכול לחם עם חתם משה, in honour of the arrival of Yitro they all came to the meal. There was no need to mention that Moses also participated as the meal was served in his tent, he being the host.

עולה וזבחים לאלוקים. This was intended as a symbol of Yitro’s acceptance of the yoke of Heaven. A similar symbolic acceptance is found in Kings II 5,17 when the general Naaman undertakes never again to offer such sacrifices as עולה וזבחים to any deities other than the G’d in heaven.

לאכול לחם עם חותן משה, to rejoice with him on his entry into the select circle of those who serve the only true G’d, the Creator of the universe. He joined the ranks of those who are described by David in Psalms 149 as ישמח ישראל בעושיו, “Israel rejoices in Him Who has proved to be its Creator.”

לפני האלוקים. in front of the altar on which they offered these sacrifices. It is not clear if this was the altar Moses had built after the victory over Amalek (in the event Yitro had arrived before the revelation at Mount Sinai) or if it had been another altar whose erection had not been especially mentioned by the Torah. At any rate, the consuming of the offerings known as זבחים, “meat-offerings,” took place within view of the altar.