My father has developed racing heartbeat. What is it exactly?

Usually arrhythmia. An arrhythmia is a disturbance in the heart beat and can be fast or slow. The most common types start in the upper portion, or atria, and are called supraventricular tachycardias–which include atrial fibrillation, atrial flutter and atrial tachycardias. Any of these can be very fast and are treated with medicines to slow the heart rate and can be caused by hypertension or other causes.