Numbers 18:17

But the firstling of a cow, or the firstling of a sheep, or
the firstling of a goat, thou shalt not redeem
That is, shalt not take the redemption price for them, but the creatures themselves; which they, the priests, were to take for their own use:

they [are] holy;
separated to the Lord, and therefore not to be used in common by men, but were sacred to the Lord, or became the property of his priests; see ( Deuteronomy 15:19 ) ; how they were to be made use of follows:

thou shalt sprinkle their blood upon the altar;
for they were to be sacrificed, and their blood used as in other sacrifices:

and shalt burn their fat [for] an offering made by fire; for a sweet
savour unto the Lord;
as the fat of other sacrifices was, and particularly of the peace offerings, which were wholly eaten otherwise, ( Leviticus 7:31 ) .