My doc says I've got cardiac arrhythmia. What does this mean?

Cardiac arrhythmia. Translated into plain english: your heart is not beating perfect like a metronome :-) this can be as simple and benign as a skipped beat (everybody has those) to life-threathening as in ventricular tachycardia or ventricular fibrillation. And then there is everything in between. Ask your doctor for a more specific definition and explanation!
Abnml heart activity. Arrhythmia ranges from the benign such as pvc's in the setting of a normal heart to the serious such as ventricular tachycardia or fibrillation in the setting of significant heart disease. The latter are both ventricular arrhythmias. Te are a host if atrial arrhythmias as well. You need to get specifics about your arrhythmia.
Depends. This is an irregularity of your heart beat and can range from mostly harmless to life-threatening. You need to speak to your primary care doctor to determine which variation you have and the appropriate treatment (or no-treatment) options. Many of these may even need referral to a cardiologist (heart specialist) but speak to your primary care physician to determine the correct course of action.