They are holy, to wit, in a peculiar manner, and higher degree than other beasts, consecrated to a holy use, even to be sacrificed unto God, and not to be redeemed nor alienated to any other use. Compare Deuteronomy 15:19.
Related Commentaries of Numbers 18:17
Numbers 18:17
17 But the firstling of a cow, or the firstling of a sheep, or the firstling of a goat, thou shalt not redeem; they are holy: thou shalt sprinkle their blood upon the altar, and shalt burn their fat for an offering made by fire, for a sweet savour unto the LORD.