What exactly is an arrhythmia? Is it a cause for concern?

Arrhythmia. First you need to diagnose what the arrhythmia is and then try to determine what might be the cause in the patient. The treatment follows from the evaluation of the what and why of the abnormal rhythm. It may be necessary to see a cardiologist or a special cardiologist called electrophysiologist.
Many things. An arrhythmia is a broad term for any irregularity in electrical conduction in the heart. You need to be more specific.