Basically, it was talking about one of the most prevalent forms of anemia, iron-deficiency anemia.
Blood is the body's delivery system.
It carries oxygen and other nutrients around the body, delivering them to the cells that need them. At the same time, the blood picks up and takes away the waste products. One of the most important things the blood does is carry oxygen from the lungs to every cell, of every muscle and organ. Without that oxygen, cells could not operate properly. Anemia basically keeps the red blood cells from delivering enough oxygen.