Select the soundsource you want to monitor in your OS and click the Go button.
You could select your microphone to listen to your voice. Or pick the line-in to react to music coming from an external source. Or just pick "Stereo-mix" to listen to sound coming from your computer.
If your browser asks for access to your Microphone, accept to make it work.
You can set most browsers to full screen by pressing F11.