Hardness is dissolved calcium and magnesium.
What it is
Hardness is the calcium and magnesium a water has picked up from limestone and dolomite on its way to the well or the plant. It is reported either in grains per gallon or in milligrams per liter as calcium carbonate, and one grain per gallon is about 17.1 milligrams per liter. Most of Michigan sits on carbonate rock, so water here usually carries some.
What it usually does
Those two minerals leave the water wherever it is heated or allowed to dry, which is why hardness usually shows up as scale on a kettle element and spots on glassware. They also bind to soap ahead of the dirt, so a hard water usually takes more soap for less lather and leaves a film behind.
What it does not prove
A hardness number says nothing about iron, bacteria, nitrate, arsenic, or anything else. Hard water is a nuisance measurement rather than a health one, and a low number does not mean the rest of a panel is clean. It also does not say whether a softener is the right size, because that depends on how much water the house uses as well as how hard it is.
When to test
Usually worth a number before anybody sizes a softener, after moving into a house on a well, and again if scale or lather changes noticeably. A drop test reads hardness in minutes, so this is one of the few numbers that usually does not need a lab.
See what this usually means for your water.
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