Atrial fibrillation is the most common heart arrhythmia in athletes. The best explanation of atrial fibrillation, in my opinion, is from the Athlete’s Heart Blog:
One simple way of looking at it is to realize that when you are in atrial fibrillation the atria (plural of atrium – the top two chambers of the heart which help fill the ventricles) are beating so fast it is as if they are not being at all, so in other words an individual who is in atrial fibrillation has had the misfortune of going from a four chamber heart to a two chamber heart. This reduces cardiac output to a certain extent, but does not necessarily exclude one from athletic activities. But it makes a runner who was previously a back of the pack runner into a runner who finishes right ahead of the people who are walking the race.