After Korach’s failed coup, Hashem reiterated the prominence that Ahron and his descendants would have. They would always be at the service of the Jewish people, guiding religious practice:
כל תרומת הקדשים אשר ירימו בני־ישראל ה נתתי לך ולבניך ולבנתיך אתך לחק־עולם ברית מלח עולם הוא לפני ה לך ולזרעך אתך – All the gifts that the Jewish people set aside for Hashem, I give to you, to your sons and daughters, as a due for all time. It shall be an eternal covenant of salt before Hashem and for you and your descendants as well. (18:19)
The covenant of salt is an expression of trust and friendship. Calling the covenant after salt calls to mind how the covenant is eternal.
But if it’s eternal, what does salt add to the expression?
Rabbi Shlomo Farhi explains that the comparison is literal as well.
The property of salt is not just that it never spoils, but that it enhances and draws out the properties of what it interacts with.
Ahron was the paragon of public service. What he did for others was he brought people together, and brought out what was best in them. Life in service of others is what made him so special.
The comparison to salt evokes a contrast to Korach, who was only in it for himself, not for others.
The mark of greatness is being there for others even when it’s a thankless task.