John Gill Commentary Numbers 18:29

"Out of all your gifts ye shall offer every heave-offering of Jehovah, of all the best thereof, even the hallowed part thereof out of it." — Numbers 18:29 (ASV)

Out of all your gifts you shall offer every heave offering of
the Lord
Not only out of their tithes, but out of whatever gifts were given by the Israelites to them, and out of the produce of the fields which belonged to their cities, (Numbers 35:4); the Levites were to take a tenth, and bring it to the Lord as a heave offering; that is, they were to give it to the priests, to testify their gratitude for everything they had, and to sanctify the rest to them, and confirm their right and title to it, and give them the free use of it:

of all the best of it, [even] the hallowed part of it, out of it;
which was the tenth part, for that was holy, as Aben Ezra says, and that was to be the best of it, which was always to be given to the Lord.