Anemia means the blood can't carry oxygen as well as it should — either because there are too few red blood cells or too little hemoglobin. Common effects are fatigue, pale skin, breathlessness and a fast heartbeat.
The most common type is iron deficiency anemia, especially in women (largely from menstrual blood loss — see iron deficiency in women). Other causes include B12 or folate deficiency and chronic disease, so getting the right diagnosis matters.