A water softener operates using a process called “ion exchange” process. When mineral laden “hard” water passes through water softener resin beads an ion exchange process takes place. As the name suggests, the undesirable hard mineral ions, (normally calcium and/or magnesium) are exchanged or trade places with 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 softener resin 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. This is why all true water softeners require salt; to regenerate their ion exchanging resin.
Click here for a video detailing the regeneration process.