Pulse width modulation sensor circuit
I suspect that some things are missing from the schematic you were given.

Consider this similar circuit.







simulate this circuit &ndash; Schematic created using CircuitLab

R1 is the thermistor. R2 is a fixed resistor, whose value might be adjusted to  tune the circuit. V1 is an oscillator with triangle-wave output. CMP1 is not an op-amp, but a comparator.

Now, the output will be high for the portion of the oscillator cycle that the triangle wave output is greater than the voltage at VS, and low for the remainder of the cycle. Since the oscillator output shape is a triangle, this means the PWM output duty cycle varies linearly with VS.

Modifying the circuit or changing the oscillator waveform to make the output duty cycle vary linearly with temperature applied to the thermistor is left as an exercise for the reader.