What does Exodus 34:19-20 mean when it talks about the first born being Yahweh's and that we are to ransom them?

Answer From The Annual Reading Schedule - Week 20 Notes For Exodus: Exodus 13:15 explains that because Yahweh had slain all the firstborn of Mitsrayim (Egypt) but spared the firstborn of Yisra'el (Israel), the firstborn of Yisra'el became His. However, we also learn in Numbers 3:12, 3:41-46, and 8:18, that instead of taking the firstborn of every single tribe in Yisra’el, Yahweh instead takes all of the Lewites. However, the ransom is still required for all firstborn because the lives of the firstborn have been spared. The ransom over and above the number of the Lewites was later given to Aharon in Numbers 3:45-51. Numbers 18 and other verses also talk about the first-born of animals and gives other requirements (e.g. sprinkling the blood of first-born of clean animals on the altar – Numbers 18:17).