A defibrillator is a machine that can help when ventricular fibrillation happens in the heart of a human. A defibrillator is a life saving device.
Fibrillation happens when many different cells in the heart begin to act as pacemaker cells. This means that many thousands of cells tell the heart to beat, all at different times and with no rhythm. This confusion causes the heart to become unable to move blood around the body. This causes cardiac arrest and death.