John Gill Commentary 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; 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 savor unto Jehovah." — Numbers 18:17 (ASV)

But the firstling of a cow, or the firstling of a sheep, or
the firstling of a goat, you shall not redeem
That is, shall 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:

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

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