All the sacred gifts that the Israelites set aside for YHVH I give to you, to your sons, and to the daughters that are with you, as a due for all time. It shall be an everlasting covenant of salt [b’rit melach] before YHVH for you and for your offspring as well. — Bamidbar/Numbers 18:19
Look for other mentions of the “covenant of salt” — sometimes rendered “salt-like covenant” — and explore possible meanings of salt in the ancient world.
“You shall season your every offering of meal with salt; you shall not omit from your meal offering the slat of your covenant with God; with your offerings you must offer salt.”—Vayikra/Leviticus 2:13
“Surely you know that the LORD God of Israel gave David kingship over Israel forever — to him and his sons — by a covenant of salt.” –2 Divrei Hayamim/Chronicles 13:5)
See also: “The world can live without wine, but the world cannot live without water. The world can live without pepper, but the world cannot live without salt.” — Bialik and Ravnitzky*, p.588
In addition, there is a midrash (don’t know the source) saying that, when God separated the waters above and the waters below, the waters below objected to being so far from heaven. God promised that their salt would be part of sacrifices, rising back to heaven….Hence salting challah.
The book Teaching Torah* notes that salt is used to kasher meat, for dipping at Passover, on challah after the blessing and in the custom of bringing bread and salt into a new home. Other sources note that salt was considered permanent by the ancients. For much more on salt — including etymology, scientific references and an email promising to offer more references — visit salt.org.
* See Source Materials for complete citations and more information.