Iron deficiency anemia is a condition that leads to smaller than normal, and decreased amount of red blood cells. This is a bad thing, because red blood cells are responsible for carrying and delivering oxygen throughout the body. Smaller cells have less room to carry oxygen molecules.
In athletes, iron deficiency is probably the most common cause of anemia. Iron deficiency typically occurs when a person doesn't get enough iron in their diet, or from blood loss (injury, surgery, or female reproductive cycles). In some cases, there are hidden sources of iron loss such as sweating (not a major cause but doesn't help) and in the gut during intense exercise (some bleeding occurs in the gut with intense, long term exercise). Lastly, undiagnosed gluten sensitivity can be a reason why a person has low iron or chronic anemia without explanation.