en_tn/num/08/16.md

each male child who opens the womb, the firstborn

These two phrases mean basically the same thing and are combined for emphasize the firstborn sons. (See: rc://en/ta/man/jit/figs-parallelism)

male child who opens the womb

This is an idiom. "To open the womb" means to give birth for the first time. Here this refers to the first male child to which a mother gives birth. Alternate translation: "child who is the firstborn son of his mother" (See: rc://en/ta/man/jit/figs-idiom)