Albert Barnes Commentary Exodus 3:1

"Now Moses was keeping the flock of Jethro his father-in-law, the priest of Midian: and he led the flock to the back of the wilderness, and came to the mountain of God, unto Horeb." — Exodus 3:1 (ASV)

Jethro his father-in-law - Or “brother-in-law.” The word in the Hebrew is a word signifying a relative by marriage. When Moses arrived in Midian, Reuel was an elderly man (Exodus 2:16); 40 years later (Exodus 2:23 note), Reuel’s son, Jethro, had probably succeeded him.

The backside - That is, “to the west of the district.” Among the Hebrews, the East is before a man, the west behind him, the south and north on the right and left hand.

Desert - Or wilderness, not a barren waste, but a district supplying pasturage. The district near Sherm, on the west of the Gulf of Akabah, where Jethro may have resided, is described as barren and parched; on the west and east are rocky tracts, but to the northwest lies the district of Sinai, where the pasturage is good and water abundant. The Bedouins drive their flocks there from the lowlands at the approach of summer. From this it may be inferred that the events here recorded took place at that season.

To Horeb - More exactly, toward Horeb. Moses came to the mountain of God, that is, Sinai, on his way toward Horeb, a name given to the northern part of the Sinaitic range. Moses calls Sinai “mountain of God” by anticipation, with reference to the manifestation of God. There is no authority for assuming that the spot was previously held sacred ; but it has been lately shown that the whole Peninsula was regarded by the Egyptians as specially consecrated to the gods from a very early time.