Jamieson, Fausset and Brown on Numbers 18:2-7

2-7. thy brethren also of the tribe of Levi--The departments of the sacred office, to be filled respectively by the priests and Levites, are here assigned to each. To the priests was committed the charge of the sanctuary and the altar, while the Levites were to take care of everything else about the tabernacle. The Levites were to attend the priests as servants--bestowed on them as "gifts" to aid in the service of the tabernacle--while the high and dignified office of the priesthood was a "service of gift." "A stranger," that is, one, neither a priest nor a Levite, who should intrude into any departments of the sacred office, should incur the penalty of death.