Meaning
To be effective and efficient; deserving of one's pay.

Origin
Sodium chloride, a.k.a. salt, is essential for human life and, until the invention of canning and refrigeration, was the primary method of preservation of food. Not surprisingly, it has long been considered valuable.

To be 'worth one's salt' is to be worth one's pay. Our word salary derives from the Latin salarium, (sal is the Latin word for salt). There is some debate over the origin of the word salarium, but most scholars accept that it was the money allowed to Roman soldiers for the purchase of salt. Roman soldiers weren't actually paid in salt, as some suggest. They were obliged to buy their own food, weapons etc. and had the cost of these deducted from their wages in advance.

From: http://www.phrases.org.uk