Cardiac describes anything that's connected or related to the heart. During a cardiac exam, a doctor listens to your heartbeat and takes your pulse.
The adjective cardiac is most often used in a medical context: a doctor who operates on people's hearts is a cardiac surgeon, and an irregular heart beat is called "cardiac arrhythmia." It's common for both medical and non-medical people to call a heart attack "cardiac arrest." The word comes from the French cardiaque, which is rooted in the Greek kardiakos, "pertaining to the heart," from kardia, "heart."