The op-Amp 1 is a buffer used to amplify the input by certain scale. Op-Amp 2 and 3 are low pass filters which allow signals to pass which are below a certain cut off frequency i.e. 200Hz. The op-Amp 4 is used to revert the inverted signals (op-Amp 1 is an inverting amplifier) back to normal (non-inverted) signals. The potentiometer is used to change the amplitude of the signals.
Design: Op-Amps used are uA741. 
Resistors, capacitors are chosen as per the cut off frequency.
Cut off frequency = 1/(2*pi*R*C)
R = resistance  C = capacitance