How a water softener works:

Hard water is softened by removing the calcium and magnesium it contains. When hard water passes through the softening system’s resin bed, it exchanges the hard ions in the water for soft ones, so only softened water passes through to your home. Once the resin bed fills up with hard ions, it must be cleaned. Salt is used in the regeneration (cleaning) process and restores the resin to a “clean” state so the ion exchange process can begin again.