The idea is to use an adjustable voltage divider to set the desired temperature while a temperature dependent voltage divider provides feedback about the actual print bed temperature.
The first op-amp IC1B works as a comparator that turns on its output to 5V as soon as the non-inverting input pin sees a higher voltage than the inverting input pin.
That output controls a simple constant current sink in which the MOSFET Q1 can be the heater or the shunt resistor R1 can be replaced with higher resistance power resistors to become heaters.
IC2 is just a voltage regulator that provides a stable supply voltage for the op-amps.
