An actual, physical capacitor is not an ideal component with capacitance only. However, it can be considered an ideal capacitor in series with a resistor and an inductor. This resistance is defined as the equivalent series resistance (ESR), while the inductance is defined as equivalent series inductance (ESL). Both affect the capacitor’s total impedance
In this model:
Total Impedance = Capacitance[1/(2πfC)] + Inductance[2πfL] + Resistance(ESR)
We can see that the total impedance of the capacitor will vary with frequency. In the beginning, the device’s capacitance dominates the impedance of the capacitor. The circuit then begins to resonate at one frequency as the frequency increases. At that resonant frequency, the total capacitor impedance is approximately equal to the ESR and the impedance of the entire capacitor is at its minimum. So the ESR of a capacitor determines its lowest impedance over a range of frequencies. Low ESR capacitors usually have good performance, especially in filter applications.