Switched capacitor circuits operate by moving charges between capacitors as switches open and close. Here, n-channel MOSFETs play the role of switches that close when high clock voltage is applied to their gates.
This circuit is Parasitic Sensitive Integrator. It produces the negative of the integral of the input voltage.
en.wikipedia.org/wiki/Switched_capacitor
Two clocks control the switches. During the first clock phase the bottom capacitor samples the input voltage and holds its value. During the second phase the held value is delivered to the op-amp for processing. Low switching frequency is used for better illustration and is normally higher (relative to input signal).
