John Gill Commentary Numbers 18:9

"This shall be thine of the most holy things, [reserved] from the fire: every oblation of theirs, even every meal-offering of theirs, and every sin-offering of theirs, and every trespass-offering of theirs, which they shall render unto me, shall be most holy for thee and for thy sons." — Numbers 18:9 (ASV)

This shall be thine of the most holy things. For, as observed, there were some lighter and lesser holy things or sacrifices, and others superlatively so: and those are begun with, such as were [reserved] from the fire; those parts of sacrifices which were not burnt with fire on the altar of burnt offerings.

every oblation of theirs; which is a general word, including various offerings, the particulars follow; every meat offering of theirs: or bread offering, for they were made of fine flour and oil, and burnt on the altar, the remainder of which was eaten by Aaron and his sons, (Leviticus 6:14–18).

every sin offering of theirs; which was offered to make atonement for sin, which also belonged to the priests and their sons, excepting such whose blood was carried into the most holy place, (Leviticus 6:25–30).

and every trespass offering of theirs, which they shall render unto me; which was offered for a trespass committed; what remained of this also, when burnt on the altar, was eaten by the priest and his sons, (Leviticus 7:5Leviticus 7:6); and this sacrifice was a ram, when a trespass was committed, either in the holy things of the Lord, or in wrongs done to men, (Leviticus 5:15) (6:6); Jarchi interprets it of an offering for a trespass committed by rapine or violence to a stranger: these, all of them, it is added, [shall be] most holy for thee and for thy sons; for their use, and for theirs only.