A simple RC low pass filter, when we run this circuit with periodic pulses with pulse width less than the circuit's settling time (approximately 5 time constants), the output will in steady state averagely tracks the DC level of the input periodic pulse.
This is typically low pass filtering the signal.
And for the little triangular shape over the DC, we can think of it as a small signal integrator.
vo = int(vi) / RC; Where all the quantities are small signal quantities over a DC. 🤗🤗
A simple proof:
https://drive.google.com/file/d/11yStM4rUSktYIOcE97lCWvCbyywuRnpg/view?usp=drivesdk
