Two ways to understand bypass capacitor to reduce supply ripple
The basic capacitor-charge-voltage formula is Q = CV or, charge equals capacitance multiplied by voltage.

We know that the rate at which charge (Q) changes is current hence, assuming the capacitor value is fixed we get: -

$$I = C\dfrac{dv}{dt}$$

So, if the fluctuations (or AC or ripple) cause a bigger dv/dt then the current is bigger also. This means that rapidly changing fluctuations see a low impedance because the current is higher when fluctuations are more rapid.