The softener operates using an “ion exchange” process. When hard water contacts the cation resin beads, by passing through the softener mineral tank which have soft sodium/potassium ions attached to them, an ion exchange process takes place with the hard mineral ions, (normally calcium and/or magnesium), and during this contact, simply trade places with the soft sodium/potassium ions.
After a calculated period of use, the sodium ions are eventually depleted and are replaced by calcium and magnesium ions.
At this point, the resin then needs to be regenerated with new sodium ions, so the resin will again be able to exchange the hard for the soft. Salt, or sodium chloride, rinses through the resin beads during the regeneration of the softener, and washes the hard water ions off of the resin beads replacing them with new sodium ions.