Numbers 3:11-13 meaning

Regarding another aspect of the role of the Levites, the Lord spoke to Moses. Here, He tells Moses that He has taken the Levites from among the sons of Israel instead of every firstborn. As a result of the tenth plague (death of the firstborn, Exodus 13:15), the LORD made the firstborn of Israel His possession (Exodus 13:2). Now, He wanted the Levites as His possession "instead of every firstborn," or the first issue of the womb among the sons of Israel. Thus, the LORD declared that the Levites shall be Mine. God took all the Levites in place of the firstborn of each family throughout the tribes of Israel. In this way they also served a priestly function, standing in place of others as a sacrifice before the LORD.

The LORD again declared that all the firstborn are Mine. The basis of this was that on the day that I struck down all the firstborn in the land of Egypt, I sanctified to Myself all the firstborn in Israel, from man to beast. To be "sanctified" meant to be set apart for a specific purpose. God set apart the firstborn of all Israel for special service to Him on the occasion of the Passover (Exodus 13:1-2). He then stated yet again that they shall be Mine, referring to the Levites, who were to serve the redemptive function of redeeming all firstborn of Israel, and serving in their place. It appears this function of the Levites only applied to the firstborn humans. The firstborn beasts were still required of Israel, in part to feed the Levites.