Why would my heart arrhythmia doctor tell me to eat more salt? he did not really tell me why.

Arrhythmia & salt: In general, too much salt can lead to fluid retention and elevated blood pressure, which in turn can increase your risk of heart disease and arrhythmia. But if your sodium level is abnormally low, you may need more salt intake. Salt may be an important source of iodine if you are deficient. So, your case should be discussed with your doctor because we don't know the context.

Low blood pressure: If your blood pressure is low and you have symptoms from this such as dizziness or fainting , than inceased salt may be part of the treatment.