Water Softeners contain an ion exchange resin that exchanges sodium ions (Na+) for calcium (Ca++) and Magnesium (Mg++) ions. When the sodium is depleted, the resin needs to be regenerated. This is done by backflushing the resin with a concentrated salt (sodium chloride) solution. Sodium ions displace the calcium and magnesium ions, which are washed down the drain, and the resin is ready for another softening cycle.