And it came to pass, when Pharaoh would hardly let us go, that the LORD slew all the firstborn in the land of Egypt, both the firstborn of man, and the firstborn of beast: therefore I sacrifice to the LORD all that openeth the matrix, being males; but all the firstborn of my children I redeem.
One day, when Pharaoh wouldn't let us go, God killed all the firstborn in Egypt - people and animals. So, I offer to God every male born from the womb, but I buy back my own firstborn children.