John Gill Commentary Numbers 18:32

"And ye shall bear no sin by reason of it, when ye have heaved from it the best thereof: and ye shall not profane the holy things of the children of Israel, that ye die not." — Numbers 18:32 (ASV)

And you shall bear no sin by reason of it
Shall have no blame laid upon them, or guilt imputed to them, or punishment inflicted on them, on account of their having and enjoying their tithes and gifts:

when you have heaved from it the best of it ;
took out the tenth, and that the best, and given it to the priests; or otherwise they would be blameworthy, guilty, and liable to punishment:

neither shall you pollute the holy things of the children of Israel ;
the gifts they have given for holy uses, and so are holy; which would be the case if they ate of them, or enjoyed them, without giving the priests the tenth part of them:

lest you die ;
for it seems such a neglect was punishable with death.