A covenant of salt, i.e. a durable and perpetual covenant; so called here and 2 Chronicles 13:5, either because salt is a sign of incorruption, as being of singular use to preserve things from corruption; or because it is confirmed and ratified on their part by salt, which is therefore called the salt of the covenant, for which the priests were obliged to take care that it should never be lacking from any meat-offering, Leviticus 2:13. And this promise or privilege conferred upon the priests is called a covenant because it is given them conditionally, upon condition of their service, and care about the worship of God, and sacrifices, which were commonly accompanied with meat-offerings, and therefore with salt.
Related Commentaries of Numbers 18:19
Numbers 18:19
19 All the heave offerings of the holy things, which the children of Israel offer unto the LORD, have I given thee, and thy sons and thy daughters with thee, by a statute for ever: it is a covenant of salt for ever before the LORD unto thee and to thy seed with thee.