Move the sliders to adjust the duty cycle and the frequency.
Many electronics use a PWM (Pulse Width Modulation) to approximate an analog voltage output. By changing the Duty Cycle (the amount of time the signal is ON), the signal will have varying average output. This can be used for example to dim LED bulbs. If the frequency is fast enough, the eye cannot detect the flickering ON/OFF.
Link to stand-alone sim: Here is a link to view the sim on its own, with no distractions.