Miller Capacitance in MOSFET
Take an opamp with gain of 100,000.
Install a 100pF capacitor from VIn- to the output.
Ground the Vin+
Now apply 1 microvolt sinusoid at Vin-.
The voltage source will have to charge up that 100pF capacitor. The other end of the capacitor will be changing, by 100,000X more voltage.
Since the required charge on a capacitor is Q = C * V, here we have a huge problem, because the effective input capacitor is

(1 + 100,000) larger than what we expected.

This ratio (1 + voltage_gain) is the Miller Multiplication of the actual capacitor.