Midian is thought to have been a wilderness on the Arabian Peninsula, across the Red Sea from Egypt and the Sinai Peninsula. Wells were traditional meeting places for biblical patriarchs and their future wives, so Moses is being placed within that tradition. Note that while in this scene Moses’s father-in-law is named Reuel, in other parts of the Bible it is treated differently: later in Exodus, Moses’s father-in-law will be called Jethro, while in the book of Numbers, Reuel is named as being Jethro’s father.