Hardness is the amount of dissolved calcium and magnesium, usually expressed as mg/L (ppm) as calcium carbonate or as grains per gallon (1 gpg ≈ 17.1 mg/L). USGS describes the household effects: soap scum, spots on glass, and scale in water heaters and pipes. USGS also notes that hardness is not a health concern in that description, and that drinking water can contribute dietary calcium and magnesium.
A cation-exchange salt softener removes those ions. A salt-free conditioner does not. Whole-house carbon does not.
City starting points on this site: Phoenix publishes 10–17.6 gpg. Santa Clarita Valley confirms hardness as an aesthetic groundwater mineral issue without a number we were willing to invent from dealer blogs.
If the tap also stains orange or smells like sulfur, hardness is not the only file. Open iron and sulfur.