C. F. Keil and Franz Delitzsch on Numbers 18:25-28

Num 18:25-28 Appropriation of the Tithe. - Num 18:26. When the Levites took (received) from the people the tithe assigned them by Jehovah, they were to lift off from it a heave-offering for Jehovah, a tithe of the tithe for Aaron the priest (i.e., for the priesthood; see at Num 18:20). “Your heave-offering shall be reckoned to you as the corn of the threshing-floor, and the fulness (see Exo 22:28) of the wine-press,” i.e., according to Num 18:30, as the revenue of the threshing-floor and wine-press; that is to say, as corn and wine which they had reaped themselves.