Resin bed
The bed of tiny beads that actually softens your water.
The resin bed is thousands of tiny ion-exchange beads inside a water softener's tall tank. Coated in sodium, they trade their sodium for the calcium and magnesium in hard water as it passes through — that swap is the softening. When the beads fill up, regeneration with salt brine recharges them. A resin bed lasts 10 to 15 years, longer on clean city water, but iron and chlorine slowly foul or wear it out.