John Gill Commentary Numbers 18:8

"And Jehovah spake unto Aaron, And I, behold, I have given thee the charge of my heave-offerings, even all the hallowed things of the children of Israel; unto thee have I given them by reason of the anointing, and to thy sons, as a portion for ever." — Numbers 18:8 (ASV)

And the Lord spoke to Aaron
Having pointed out to him the duty of his office, he now informs him of the perquisites of it; or having told him what was his work, now what his wages, or what was his service, and now his maintenance:

behold, I also have given you the charge of my heave offerings, of
all the hallowed things of the children of Israel ;

such as the heave shoulder and wave breast, and firstfruits, and all other holy things in general, such as were the most holy things; as in (Numbers 18:9) ; and the lighter holy things, as the Jews call them, (Numbers 18:11) ; the charge of these was given to keep for themselves, and for their families:

to you have I given them by reason of the anointing :
because of his office as a priest, to which he was consecrated by anointing:

and to your sons by an ordinance for ever ;
see (Leviticus 7:34–36) .