The heart is an organ found in every vertebrate. It is a very strong muscle. It is on the left side of the body in humans and is about the size of a fist. It pumps blood throughout the body. It has regular contractions, or when the heart squeezes the blood out into other parts of the body.
Cardiac and cardio both mean "about the heart", so if something has the prefix cardio or cardiac, it has something to do with the heart.
Myocardium means the heart muscle: 'myo' is from the Greek word for muscle - 'mys', cardium is from the Greek word for heart - 'kardia'.