Iron comes in two forms, and the form usually matters more than the amount.
What it is
Iron is dissolved out of rock and soil and arrives in one of two states. Ferrous iron is dissolved and invisible, so the water is clear at the tap. Ferric iron has already met air and turned to rust particles, so the water looks orange on arrival. The same well can deliver both, and a lab usually reports total iron in milligrams per liter.
What it usually does
Iron usually stains anything it dries on, which is why it shows up as orange or brown marks where water drips or stands, and as yellowed laundry. It also gives a metallic taste, feeds iron bacteria, and will foul softener resin if a softener is asked to take it out beyond what it is rated for.
What it does not prove
An iron result says nothing about whether the water is drinkable. Iron is mostly an aesthetic and equipment problem at the levels found in homes. A number on its own also does not pick the treatment, because the right media depends on the form, the pH, and whether manganese or bacteria are in there too.
When to test
Usually worth measuring before buying any iron filter, and worth measuring alongside pH and manganese rather than on its own. If the water is clear at the tap and rusty an hour later, say so when the sample goes in, because that usually changes how it is handled.
See what this usually means for your water.
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