Red blood cells are the cells in the blood which carry oxygen and carbon dioxide through the bloodstream. They make up the majority of the solid portion of the blood - well over 95%. The red color is from hemoglobin, the actual molecule with a base of elemental iron that actually binds to oxygen and carbon dioxide and allows animals with bloodstreams to have cells that are not directly in contact with the environment in order to be oxygenated.
A shortage of red blood cells is referred to as anemia. There are several conditions that can lead to anemia. The standard treatment when the level of red blood cells falls too low to support the requirements of the body, either due to anemia or blood loss, is a blood transfusion.