Natural water contains calcium and magnesium ions which form salts that are not very soluble. These cations, together with the less common and even less soluble strontium and barium cations, are called together hardness ions. When the water evaporates even a little, these cations precipitate. This is what you see when you let water evaporate in a boiling kettle on the kitchen stove.

Hard water also forms scale in water pipes and in boilers, both domestic and industrial. It may create cloudiness in beer and soft drinks. Calcium salts deposit on the glasses in your dishwasher if the city water is hard and you have forgotten to add salt.

Strongly acidic cation exchange resins  used in the sodium form remove these hardness cations from water. Softening units, when loaded with these cations, are then regenerated with sodium chloride (NaCl, table salt).

Reactions

Here the example of calcium:

2 R-Na + Ca++  [--->]  R2-Ca + 2 Na+

R represents the resin, which is initially in the sodium form. The reaction for magnesium is identical.

The above reaction is an equilibrium. It can be reversed by increasing the sodium concentration on the right side. This is done with NaCl, and the regeneration reaction is:

R2-Ca + 2 Na+  [--->]  2 R-Na + Ca++​

The water salinity is unchanged, only the hardness has been replaced by sodium. A small residual hardness is still there, its value depending on regeneration conditions.

Treated water quality (residual hardness)

Co-flow regeneration: Depends on water composition

and regenerant level

Reverse flow regeneration: < 0.02 meq/l (1 mg/l as CaCO3)

Uses

Examples for the use of softeners:

Treatment of water for low pressure boilers

In Europe, most dishwashers have a softening cartridge at the bottom of the machine

Breweries and soft drink factories treat the water for their products with food grade resins

Softening the water does not reduce its salinity: it merely removes the hardness ions and replaces them with sodium, the salts of which have a much higher solubility, so they don't form scale or deposits.