Numbers 3:45 Take the Levites instead of all the firstborn among the children of Israel, and the cattle of the Levites instead of their cattle; and the Levites shall be mine: I am the LORD.
God had saved the firstborn children of Israel there in Egypt. On the horrible night, when all the firstborn Egyptians died, God had given a lamb to take their place. Now God explains that they owe Him their firstborn children. He will take a tribe, the Levites, to serve in their place. He will allow them to keep their children and their cattle. They will only be expected to take care of the tribe of Levi and their cattle.
God was very kind. He had saved all their firstborn. God could easily have expected and required them to give them their firstborn, but He knew how much they wanted their children for themselves. He made an arrangement that would meet His needs and their wants.
They could keep their children but would be expected to take care of the Levites. What a good God we serve! If you are not in the ministry, you should be supporting those that are in the ministry. That might be a good take away from this lesson.