Every thing that openeth the matrix in all flesh, which they bring unto the Lord, whether it be of men or beasts, shall be thine: nevertheless the firstborn of man shalt thou surely redeem, and the firstling of unclean beasts shalt thou redeem. (Numbers 18:15)
Two classes of the first-born that belonged to the priests were to be redeemed, that is, bought back with money:
(1) unclean animals, being unacceptable as sacrifices, and
(2) human beings.
That openeth the matrix.
That which was born first, if a male, was to be the priests’. If a female should be born first and a male next, the male was not to be the priests’, since it did not in that case open the womb (Ex. 13:2).