A water softener is an ion exchanger.
Hard water-water with a high calcium/magnesium content-enters the softener through the "In" port indicated by the green arrow. It passes through the control valve and into the tank, where it goes from top to bottom through a specially prepared resin that "softens" it.
The resin consists of specially manufactured beads that have been saturated with sodium ions. "Softening" occurs as the hardness minerals in the water attach themselves to the resin and are "exchanged" for sodium.
The softened water then enters the long center tube, called a riser, via the strainer basket in the bottom of the tank and passes upward through the riser. The water exits the softener via the control valve (blue arrow) and is sent to the home.
When the resin becomes saturated by hardness minerals, the softener automatically goes into regeneration. (The regeneration process is initiated by a timer or a meter, depending on the type of softener you purchase.) By this process the hardness minerals are washed down the drain (via a drain tube not shown in the diagram), and the resin bed is rinsed, resettled, and recharged with sodium. It is now again ready to soften your water.
The regeneration process is accomplished by passing very salty water from the brine tank through the resin.
The brine tank must remain filled with softener salt at all times so that it can regenerate the softening resin again and again.