Albert Barnes Commentary Numbers 18:19

"All the heave-offerings of the holy things, which the children of Israel offer unto Jehovah, have I given thee, and thy sons and thy daughters with thee, as a portion for ever: it is a covenant of salt for ever before Jehovah unto thee and to thy seed with thee." — Numbers 18:19 (ASV)

A covenant of salt - Compare the marginal reference. Covenants were ordinarily cemented in the East by the rites of hospitality; salt was the obvious token, as it is an ingredient in virtually all food. It indicates perpetuity: compare the note on Leviticus 2:13.