Atrial fibrillation is a potentially life-threatening condition if it is allowed to persist.  It causes the upper heart chamber (atrium) and/or chambers (atria) to beat at 500-600 times per minute.  This is not compatible with normal mechanical function of the atrium.  As a results, the atrium or atria quiver like a bag of worms rather than function as they should moving blood flow in a coordinated fashion by contracting immediately prior to the larger chambers (ventricles).  The lack of mechanical contraction results in a stagnation of blood flow within the atrium and resultant blood clot formation.