This 3D animation by The Human Atlas explains Atrial Fibrillation. 

The heart is a muscle that contracts in rhythmic sequence for the duration of our lifetime. Each beat is stimulated by an electrical signal that is generated by the heart's conduction system. A normal heart beats 60 to 100 times per minute. Sometimes, a problem with the conduction system causes the heart to beat too fast, too slow, or to have an erratic or irregular beat. A test, called an electrocardiogram, or EKG, can measure and record the heart's electrical activity.

In a normal heartbeat, the heart's signal follows a specific pathway through the heart. The signal begins in the sino-atrial node, or SA node, located in the right atrium. The SA node triggers the atria to contract, pushing blood into the ventricles. The electrical signal then travels through the atrio-ventricular node, or AV node, and into the ventricles. This signal now causes the ventricles to contract, pumping blood to the lungs and body.

Atrial fibrillation is a type of arrhythmia, or abnormal heartbeat, that is caused by erratic electrical signals originating from the atria. During this rhythm disturbance, the normal, coordinated contractions between the atria and ventricles become compromised, interfering with the hearts ability to efficiently deliver blood to the body.

In people who experience atrial fibrillation, the SA node fires rapidly, causing the atria to contract erratically. The irregular contractions do not fill the ventricles with blood properly, causing the ventricular contractions to also become erratic. The heart rate may increase to 100 to 175 beats per minute or more.

Atrial fibrillation can cause fainting, weakness, and may lead to blood clots and other complications. The condition can be treated with medication or with surgery. In some patients, a pacemaker may be inserted in order to regulate heart rhythm.