Before Aaron and his sons, i.e. put them into the power of Aaron and his sons, to employ them in holy ministrations; for so that phrase is sometimes used, as Genesis 13:9, the land is before thee, i.e. in thy power, to use or enjoy it. Or setting the Levites before them did signify the giving the Levites to them, or to their service. For an offering unto to the Lord; for to him they were first properly offered, and by him given to the priests in order to his service.
Related Commentaries of Numbers 8:13
Numbers 8:13
13 And thou shalt set the Levites before Aaron, and before his sons, and offer them for an offering unto the LORD.