ESR is the equivalent series resistance of a capacitor. An ideal capacitor would have only capacitance. As you remember, all conductors have resistance. In a capacitor, there are multiple conductors like the wire leads, the film and the electrolyte. The resistances of all the conductors contribute to the capacitor's series resistance. It's essentially the same as having a resistor in series with an ideal capacitor. Capacitors with relatively high ESR will have less ability to pass current from its plates to the external circuit (to the amplifiers in the case of large capacitors in car audio). Low ESR is desirable when using a capacitor as a filter.