“stubbornly refused.” The Hebrew is qashah (#07185 קָשָׁה), see commentary on Exodus 4:21.
“that opens the womb.” This is an idiom for being the firstborn. The firstborn “opens the womb.”
“but all the firstborn of my sons I redeem.” The firstborn male children could not be offered as sacrifices, so they had to be redeemed—bought back from Yahweh with money. The redemption price is not given in Exodus, but is given in Numbers 18:15-16 as five shekels of silver.