Funny how a diagnosis doesn't mean much till you have it. What is atrial fibrillation?

Electrical chaos. The electrical impulses in the upper chambers of your heart (mostly originating from the left side) are completely chaotic leading to a quivering motion of these chambers instead of their typical, coordinated pumping function. The lower chambers beat erratically, as the electrical impulses bombard them in a haphazard way. See a cardiologist or an elelctrophysiologist.
An irregular rhythm. Instead of responding to a single pacemaker in the upper part of the heart, many pacemakers appear and the lower part of the heart can respond to any of them. This results in an irregular rhythm which makes the pumping action of the heart less efficient. It can be caused by alcohol, valvular heart disease, congestive heart failure and coronary artery disease. Risk of stroke is increased.