Ex 4:22 "Then you shall say to Pharaoh, 'Thus says the LORD: "Israel is My son, My firstborn.

23 "So I say to you, let My son go that he may serve Me. But if you refuse to let him go, indeed I will kill your son, your firstborn."'

Pharaoh ought to know that Israel was not just any other ordinary nation. Rather, Israel is a nation that has a special relationship with God. Israel is not a slave but God's firstborn. The word "firstborn" is a term of position and rank. He is destined to be supreme among the nations.  

Since Israel is considered God's firstborn, refusal to release them would result in corresponding punishment.

This is Moses' way of teaching Israel their status before God. They are God's firstborn son.