FRA composite capacitor measurement
A good power supply delivers DC with very low AC impedance. The AC impedance at any particular frequency determines the ripple. As an example switching power supply might operate at 600 kHz, and so draws considerable power at 600 kHz and harmonics. The voltage ripple on the input supply will propagate back to what ever the power supply is connected to, which may cause problems with EMC, injection of the voltage ripple into other circuitry (especially if PSRR is poor), which can have unintended consequences. As an example, say the power supply draws 1A, and the input supply had an impedance of 500 mOhms, then a voltage ripple of 500mV can be expected.
This video shows that parallel combinations of appropriate capacitors can reduce supply impedance over a much larger frequency range.