Quick Answer
A water softener uses ion-exchange resin: hard-water calcium and magnesium ions are swapped for sodium (or potassium) ions as water passes through. It addresses hardness for the whole home — unlike a point-of-use filter, which does not remove hardness.
Problem
People often expect a filter or softener to do the other's job. Knowing the mechanism clarifies which device solves which problem.
Product Recommendation
ZOOSHEN is point-of-use shower filtration — it does not soften water. For hardness, research softeners; for the shower specifically, ZOOSHEN targets chlorine/chloramine. See /guides/shower-filter-vs-water-softener.