Apparently Moses had sent Zipporah and the kids back to her father’s at some point during his contest of wills with Pharaoh, because around this time, her father Jethro-Reuel decided that things seemed safe enough for him to return his daughter to her husband. So he packed up Zipporah and the kids and headed out into the wilderness where Moses was camped.
When Jethro-Reuel caught up with Moses, they hugged and spent the evening catching up on everything that had happened. When Moses told Jethro-Reuel all his adventures, Jethro-Reuel was all, you are one lucky SOB! Let’s burn some sacrifices to God.
Next day, Moses sat down to listen to all the complaints and disputes of the Hebrews and to judge between them, as was his usual morning routine. Jethro-Reuel was all, what are you doing, son? You’re going to wear yourself out! Let me give you some advice–divide up the people into constituencies–tribes you might say–and then appoint lesser chiefs and judges over them to hear all their petty disputes. That way you only have to deal with the really important or really difficult problems. Moses was all, gee, that’s a swell idea! and took the advice.