This circuit is a basic low pass filter (LPF) array (of 2) with an amplifier which filters out the lowest frequency sine wave from a square wave. Due to it being a simulation, it needs a triggering half a minute before it can come to the correct output. Thus open the key after 10 periods of input wait for it to normalise.
A square wave can be defined as a specific Fourier series, meaning, it can be constructed as a series of sine waves with specific coefficients and frequencies. Thus, a filter can be used to extract certain sine waves from the square wave. So a low pass filter would be enough to extract the lowest frequency sine wave (the fundamental sine wave).
The equivalent of a low pass filter in analogue electronics would basically be a resistor in series and a capacitor in parallel to the output. Thus, two pairs of these are used to filter the fundamental sine wave. Then the result is amplified with an op-amp.
Also notice the phase delay of the sine wave due to the two filters used.
