'and "fixed them...' Not really. The condition that makes the heart fibrillate cannot be corrected. It's a permanent, and often progressively worse, condition. But there is a 'remedy'. The remedy is to literally 'block' those signals from making the atrium muscles contract. This is achieved by ablating the tissues surrounding the area where the extra signals come from. In about 90% of al initial diagnoses of AF, that area is going to be inside one or more of the four pulmonary vein ostia, or their 'mouths,' where they empty freshly oxygenated blood returning from the lungs to the heart. The left atrium receives oxygenated blood and pumps it down, through the one-way mitral valve, into the left ventricle below the valve. The ventricle then pumps it up the curving and large aorta where other vessels branch off and take the freshened blood all over one's body.
When AF happens, extra signal bleeds out into the pulmonary veins from the normal and original path across the septum, the thick wall between the four chambers. Endothelial tissue lining the left atrium migrates into the mouths , the ostia, of the pulmonary veins and can pick up the extra signal. When that happens, the signal propagates across the rest of the endothelial tissue lining the entire left atrium, and that wave of signal causes a contraction. Except, it's out of sequence! And worse, the atrium just finished contracting a fraction of a second ago from the normal signal. Now it is forced to contract again, and it hasn't filled yet from the pulmonary veins. Even worse, being empty, the powerful ventricle below it tries to inflate it with blood, except that darned mitral valve is not meant to open for that. This puts an enormous strain on the mitral valve and can cause it to prolapse over time, usually several months if the AF persists a long time.