Meaning

Those of great worth and reliability.

Origin

The phrase 'the salt of the earth' derives from the Bible, Matthew 5:13 (King James Version):

Ye are the salt of the earth: but if the salt have lost his savour, wherewith shall it be salted? it is thenceforth good for nothing, but to be cast out, and to be trodden under foot of men.

While it is good to find a clear source for a well-known phrase, it isn't clear why salt was chosen to represent goodness. The positive meaning in this phrase conflicts with many other uses of the word salt, which generally express negative concepts. For example, in the Middle Ages, salt was spread on land to poison it, as a punishment to landowners who had transgressed against society in some way.

It seems that the 'excellent' meaning in 'the salt of the earth' was coined in reference to the aristocratic and powerful of the earth, i.e. those who were 'above the salt'.

The phrase was first published in English in Chaucer's Summoner's Tale, circa 1386, although Chaucer undoubtedly took his lead from the Bible:

Ye been the salt of the erthe and the savour.