put a triangular waveform, say with a minimum voltage of 1V and a maximum voltage of 3V on the inverting input of an op-amp. This gives a total peak-to-peak voltage of 2V on what is called the oscillator. Now take a variable DC voltage and apply it to the non-inverting input of the same op-amp. Start the DC voltage at 0V and slowly ramp it up. The op-amp output will be low until the DC voltage reaches 1V. At this point the output will begin to show very short pulses. As the DC voltage is increased, the width of these pulses keep increasing. once the DC voltage gets to 1.5V, the width of the pulses will be 25% of the period. At 2V, the pulse width is at 50% of the period. At 2.5V, the pulse width is 75% of the period. The width of the pulse in relation to the period is called the duty cycle. Once the DC voltage gets to and exceeds 3V, the output of the op-amp goes high, or has a 100% duty cycle.
This is basically how open loop PWM works. Close the loop by feeding back the output and comparing it to a reference. The output of this comparison (the error) will be the varying DC voltage from above.