The Literal Commandment
So you shall attend to the obligations of the sanctuary and the obligations of the altar, so that there will no longer be wrath on the sons of Israel. (Numbers 18:5)
This commandment is literally for Aaron and his sons (the Aaronic priests) to attend to the obligations [Hebrew: mishmereth- guard, watch, charge, or function] of the sanctuary. The last part of the verse ("so that there will no longer be wrath on the sons of Israel") refers to the rebellion of Korah (Numbers 16 & 17) that was settled by G-d in Numbers 17:8.