The 10 Ohms is the source´s internal resistance and has a high influence on this circuit´s frequency response. The input signal to the filter is probed after this part. The two capacitors must be trimmed to overlay their cutoff frequencies to achieve a steeper decay. If their cutoff frequencies are too far away from each other, the circuit behaves like two 1st order low passes in series. The 100k Ohms is the load (input impedance of the following circuit). The circuit is trimmed to 100 Hz cutoff frequency (@-45° phase angle).
