a square wave consists of a fundamental sine wave component of the same frequency, together with odd harmonics. A 600 Hz square wave thus consists of 600 Hz, 1800Hz, 3000 Hz, ... etc. components.

Apply the LP filter. The only frequency component of the square wave signal lying within the filter passband is the 600 Hz fundamental, and you should therefore find that the filter output is sinusoidal, with a frequency (600 Hz) matching that of the square wave itself.

Now generate a 330 Hz square wave signal, and apply the LP filter as before. The third harmonic component of the square wave, at a frequency of 990 Hz, now lies within the filter passband as well as the 330 Hz fundamental. You should find that the filter output resembles a sine wave at the same frequency (330 Hz) as the square wave, but now with an added third harmonic (with an amplitude only one-third that of the fundamental) which produces distinctive dips in the peaks of the waveform, and also makes the zero-crossings of the waveform slightly steeper than for a pure sine wave.

Now, without altering the input signal (the 330 Hz square wave should be displayed in the upper graph), apply the BP (band pass) filter. This time, only the third harmonic component at 990 Hz lies within the filter passband (around 1000 Hz), and the filter should therefore select out the 990 Hz sine wave component of the square wave. This can be verified by confirming that there are three cycles of the sine wave shown in the lower (output signal) graph for each cycle of the square wave plotted input in the upper graph.