It is a series resistor inside capacitor, using 100kHz to remove the impedance 1/(2*pi*F*C).
The impedance become small, and we can then measure the true series resistor value.
Using this theory, we can measure the capacitor is bad/ damaged or good in condition.
Our ESR meter only applies less than 15mV DC or peak to peak on a good capacitor, as a result we can use it as in circuit test.
During repairing TV, LCD, Audio board, etc. we can in circuit testing the capacitor is good or not.