A Commentary, Critical, Practical, and Explanatory on the Old and New Testaments, by Robert Jamieson, A.R. Fausset and David Brown  at sacred-texts.com
num 18:1THE CHARGE OF THE PRIESTS AND LEVITES. (Num 18:1-7)
the Lord said unto Aaron, Thou and thy sons and thy father's house with thee shall bear the iniquity of the sanctuary--Security is here given to the people from the fears expressed (Num 17:12), by the responsibility of attending to all sacred things being devolved upon the priesthood, together with the penalties incurred through neglect; and thus the solemn responsibilities annexed to their high dignity, of having to answer not only for their own sins, but also for the sins of the people, were calculated in a great measure to remove all feeling of envy at the elevation of Aaron's family, when the honor was weighed in the balance with its burdens and dangers.
num 18:2thy 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.
num 18:8THE PRIESTS' PORTION. (Num 18:8-20)
the Lord spake unto Aaron, Behold, I also have given thee the charge of my heave offerings--A recapitulation is made in this passage of certain perquisites specially appropriated to the maintenance of the priests. They were parts of the votive and freewill offerings, including both meat and bread, wine and oil, and the first-fruits, which formed a large and valuable item.
num 18:14Every thing devoted in Israel shall be thine--provided it was adapted for food or consumable by use; for the gold and silver vessels that were dedicated as the spoils of victory were not given to the priests, but for the use and adornment of the sacred edifice.
num 18:19it is a covenant of salt--that is, a perpetual ordinance. This figurative form of expression was evidently founded on the conservative property of salt, which keeps meat from corruption; and hence it became an emblem of inviolability and permanence. It is a common phrase among Oriental people, who consider the eating of salt a pledge of fidelity, binding them in a covenant of friendship. Hence the partaking of the altar meats, which were appropriated to the priests on condition of their services and of which salt formed a necessary accompaniment, was naturally called "a covenant of salt" (Lev 2:13).
num 18:21THE LEVITES' PORTION. (Num 18:21-32)
I have given the children of Levi all the tenth in Israel for an inheritance, for their service which they serve--Neither the priests nor the Levites were to possess any allotments of land but to depend entirely upon Him who liberally provided for them out of His own portion; and this law was subservient to many important purposes--such as that, being exempted from the cares and labors of worldly business, they might be exclusively devoted to His service; that a bond of mutual love and attachment might be formed between the people and the Levites, who, as performing religious services for the people, derived their subsistence from them; and further, that being the more easily dispersed among the different tribes, they might be more useful in instructing and directing the people.
num 18:23But the Levites shall do the service of the tabernacle of the congregation: they shall bear their iniquity--They were to be responsible for the right discharge of those duties that were assigned to them, and consequently to bear the penalty that was due to negligence or carelessness in the guardianship of the holy things.
num 18:26the Levites . . . offer . . . a tenth of the tithe--Out of their own they were to pay tithes to the priests equally as the people gave to them. The best of their tithes was to be assigned to the priests, and afterwards they enjoyed the same liberty to make use of the remainder that other Israelites had of the produce of their threshing-floors and wine-presses.
num 18:32ye shall bear no sin by reason of it, &c.--Neglect in having the best entailed sin in the use of such unhallowed food. And the holy things would be polluted by the reservation to themselves of what should be offered to God and the priests.