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.
Related Commentaries of Numbers 18:24
Numbers 18:24
24 But the tithes of the children of Israel, which they offer as an heave offering unto the LORD, I have given to the Levites to inherit: therefore I have said unto them, Among the children of Israel they shall have no inheritance.