During the middle ages, the pervasive theory regarding how the human body worked (specifically bodily fluids) was that the body contained four 'humors': blood, phlegm, black bile, and yellow bile. This theory was known as Humorism. Most physicians during the time period believed that many illnesses was caused by an imbalance of one of the four humors, aka the patient had 'bad blood.'
The solution to this was a procedure known as bloodletting, wherein blood would be drained from people thought to be suffering from an imbalance of the humors.
As a procedure, bloodletting was conducted in a number of ways, but the two most popular were a) the attachment of medical leeches to the patient and b) incisions on the patient's body, usually the arms, in order to drain the blood.