Saltwater pool

Saltwater pools use dissolved salt (3,000-6,000 ppm) as a store for the chlorination system. The chlorinator uses electrolysis to break down the salt (NaCl) and release the chlorine into the water. As such, a saltwater pool is not actually chlorine-free; it simply utilizes a chlorine generator instead of direct addition of chlorine. When chlorine levels are low in the pool, one possible cause is low salt (others can be higher-than-normal chlorine demand, low stabiliser or mechanical issues with the generator itself.) Salt count can be lowered due to splash out, backwashing, and dilution via rainwater.

Article courtesy of wikipedia