The medical term "fibrillation" is used to describe rapid, uncoordinated movements of individual muscle fibres resulting in useless contraction.
In ventricular fibrillation, the heart literally looks like a mass of worms twitching around, whilst the pumping action of the heart pretty much stops. VF is a life-threathening situation and defibrillation is required promptly or the patient WILL DIE. Attempted heroic efforts at CPR will only delay the inevitable if defibrillation is not available.
In atrial fibrillation, the muscles of the atria do not contract in synchrony. This matters less than ventricular fibrillation because the work of the atria are not anywhere near as important as the work of the ventricles of the heart. The heart can still function as a pump if the ventricles work.
Atrial fibrillation does have its own dangers though, the worst being the predisposition to forming dangerous blood clots (which can, amongst other things, embolize to cause strokes) due to the turbulent flow in the atria. Many people with long-standing atrial fibrillation are treated with long-term anticoagulation therapy, usually involving warfarin.