Moses' father-in-law Jethro, the priest of Midian, heard about everything that God had done for Moses and His people Israel, and how the Lord had brought Israel out of Egypt.
References for Exodus 18:1
Now Jethro, Moses' father-in-law, had taken in Zipporah, Moses' wife, after he had sent her back,
along with her two sons, one of whom was named Gershom (because Moses had said, "I have been a stranger in a foreign land")
References for Exodus 18:3
and the other Eliezer (because [he had said,] "The God of my father was my helper and delivered me from Pharaoh's sword").
References for Exodus 18:4
Moses' father-in-law Jethro, along with Moses' wife and sons, came to him in the wilderness where he was camped at the mountain of God.
He sent word to Moses, "I, your father-in-law Jethro, am coming to you with your wife and her two sons."