This is generally the level of calcium and magnesium ions present in the water although other cations such as strontium, iron, manganese and barium can also contribute. Hard water requires more soap than soft water to obtain a lather or soapy bubbles. It can also cause scale to form on hot water appliances, air conditioners, etc.
While some degree of hardness is present in just about all water supplies, many supplies using bore water have very high hardness levels requiring installation of a water softener or water conditioner.
Water softeners remove the calcium and magnesium ions and exchange them with sodium ions or salt. Adding salt to the water supply can in many cases be undesirable along with the water volume and residual salt that is back flushed down the drain or out into the ground every time the water softener regenerates/ cleans. This regeneration maybe required every 3-7 days.
Water conditioners on the other hand do not remove the calcium or magnesium ions they merely change the polarity or change of the ions so they do not attract each other, staying in solution rather than coating/ scaling up equipment.