Yisro, the priest of Midyan, Moshe's father-in-law, heard all that Hashem had done for Moshe and His people, Israel, that Hashem had brought Israel out of Egypt.
The Torah gives us the title of Yisro, as though it was an honor and privilege. We know of course, that a priest of Midyan is not a prestigious title at all. Why did the Torah give them this embarrassing title?
The Ohr Hachaim suggests that the Torah means to tell us that Yisro had two titles at the time, and he relinquished both to join Klal Yisroel and serve Hashem. He was both the Priest in a high position in Midyan, and he was also a strong devotee of the idols.
As Moshe’s father-in-law, he had no honor or respect. When Tziporah married Moshe, he was a simple person, they had no way of knowing what he would end up becoming. Usually, powerful people choose spouses and partners based on what they have achieved already. Yisro was willing to permit his daughter to marry Moshe, against his position as priest of Midyan.
As far as his place in the world as a devotee of the idols, he announced עַתָּה יָּדַעְתִּי כִּי גָּדוֹל ה' מִּכׇּל הָּאֱלוקים - Now I know that Hashem is greater than all the gods. He renounced his connection to the idols and cleaved to Hashem.
Another reason he is called the priest of Midyan is to show us where he received his information from. He was a leader, and he heard what was going on in the world.
