An arrhythmia is an abnormal heartbeat. A healthy heart beats 60 to 100 times a minute. That's an average of 100,000 beats a day. Abnormalities in beat include the heart beating too fast, too slow, or in an irregular rhythm. Because your heart is constantly beating, it's not surprising that your heart might not beat perfectly every single time. Thus, as Dr. Sherman explains to Scientific American, while heart irregularities are rare, arrhythmias are most likely normal.
The heart has four chambers: the right atrium and right ventricle, and the left atrium and left ventricle. The right atrium receives oxygen-poor blood and sends it down to the right ventricle. This ventricle then pumps the oxygen-poor blood through the pulmonary artery to the lungs. The left atrium receives the now oxygen-rich blood and pumps it to the left ventricle. The left ventricle pumps the blood through the main artery in the body, called the aorta, which distributes the blood through systemic circulation. When the ventricles pump blood, the heart beats. If the heart is not pumping blood effectively, the heart won't beat properly.
Sources: Scientific American, IvyRose Holistic, NIH (National Heart, Lung, and Blood Institute)