What is salt used for in a water softener?

For the resin to continue softening the water, it must be “regenerated”. Regeneration consists of “washing” it with salt water (brine) to remove all the calcium and magnesium ions it has captured from hard water. The sodium ions in the brine, present in large quantities, gradually replace the calcium and magnesium ions. The resin becomes saturated with sodium ions once again and is therefore effective at softening the water.