The first opamp buffers the input to provide a high input resistance, this feeds the circuit built around the second opamp which is the actual Schmitt trigger. The voltage source connected to the -Ve input of this opamp determines the midpoint of the input range, 6 volts in this case. The opamps have been set up to behave like an LM324 / LM358 connected to a single + 12 volt supply, the 68 K resistor determines the width of the input threshold window aka its hysterisis, it gets smaller as the value of this resistor is increased
