Equivalent Series Resistance (ESR) is a measure of the internal resistance inherent in a capacitor or an inductor. It represents the total resistance present in series with the ideal reactive component of the component. In capacitors, ESR is mainly due to the resistance of the conducting materials and the electrode-electrolyte interface. In inductors, it is primarily due to the resistance of the wire winding. ESR affects the performance and efficiency of electronic circuits, especially in high-frequency applications, where it can cause energy losses and affect the circuit's stability and response.