A transfusion is when doctors put blood into a patient's vein. An accident victim might need a transfusion if he's lost blood.
Besides accidents, there are other medical conditions that require blood transfusions, such as heart disease and some kinds of surgery. The blood a patient receives during a transfusion usually comes from donations given by healthy people. You can use the noun transfusion for any transfer of liquid, like the pouring of lemonade from a glass pitcher to a plastic one, though it's most often used to describe the medical procedure.