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