Albert Barnes Commentary Numbers 18:24

"For the tithe of the children of Israel, which they offer as a heave-offering unto Jehovah, I have given to the Levites for an inheritance: therefore I have said unto them, Among the children of Israel they shall have no inheritance." — Numbers 18:24 (ASV)

Here the tithes (and in Numbers 18:26 the priestly tithes) are to be dedicated to their purpose by the ceremony of heaving them to the Lord. The tithes, being solemnly set apart for sacred purposes, became virtually a heave-offering, like the gifts for the tabernacle (Exodus 25:2).