Cardiac arrest refers to a sudden stop in heart function, where the heart is no longer able to pump blood effectively. This can be caused by various factors such as an abnormal heart rhythm (arrhythmia), heart attack, or other medical conditions that affect the heart's electrical system. When this happens, the brain and other vital organs do not receive oxygen-rich blood, leading to unconsciousness and potentially death if not treated promptly with CPR and defibrillation.