Saying a person is worth his salt means the person is good at his job and worthy of the pay he receives. It is considered a compliment.
The word "salary" derives from the Latin "salarium," meaning "salt money," and salarium referred to an aspect of Roman legionaries' pay. This linkage was due to the importance of salt; it not only served for seasoning but also for food preservation. Salt is essential to life and, being hard to obtain due to limited transportation means, it was a highly valued trade commodity. This substance even served as currency in some cultures. Recognizing the importance of salt for health, rulers imposed heavy salt taxes on their people, from as early as ancient Chinese and Roman eras. Salt taxes likely funded many large projects, including Napoleon's foreign wars and the Erie Canal in New York State.