Iron is an essential mineral within human nutrition, being key to the formation of haemoglobin for red blood cells and for myoglobin needed in muscles for respiration. Humans have about 4 - 5 g of iron in the body, most of which is "haem" in the blood. We need about 10 mg of iron daily, of which only 0.5 - 1.5 mg is absorbed each day.
Although iron is found in unrefined foods - liver, whole wheat, fruit and vegetables, it is often not absorbed well and is inadequate in most commercially available breads. Vitamin C can help with iron uptake. For reference on a per 100 g of foods: liver contains 6 - 14 mg; cereals up to 9 mg; nuts 1 - 5 mg; eggs 2 - 3 mg; meat 2 - 4 mg.
Iron is absent from, or inadequately found, in white flour, milk and sugar. So white flour is generally enriched, i.e. added back into it - in the USA, roughly 2.9 mg is added to each 100 g of white flour. Fortified cereals provides about ⅓ of iron in the British diet.