Salt is taken for granted in these times but not back when Jesus was speaking. Just to mention salt meant a multitude of things to the people who heard what he said.
Salt also functions as a preservative, something that was especially important to people living in a hot, dry climate without the benefit of refrigeration. The Old Testament referred to a "covenant of salt" that the Lord made with the people of Israel as a permanent condition (Numbers 18:19). Thus, salt, the preservative, signified the everlasting contract between the Lord and his people.
Mark: A Devotional Commentary
(The Word Among Us)