Measure or estimate the distance between your speakers and set the Speaker Separation slider accordingly. (Uncheck the Metric Units checkbox if you want.) Then turn on the Sound checkbox. The applet will play a sine wave out of both speakers, which will create an interference pattern hopefully quite similar to the one shown. The speakers are shown as blue dots. Plug one ear and move your head around the room (at speaker level) to see if you can pick up the variations in sound intensity. They should roughly match the interference pattern, although there may be many differences because of reflections off walls and objects; also this applet uses a simplistic point source model for the speakers, which will not match reality very well.
If you have a stereo sound card, then check the Stereo checkbox. You can use the Balance slider to verify that the stereo is working. With stereo turned on, you can adjust the relative phase of the two speakers. By default, the speakers are 180 degrees out of phase, so the sound intensity will be low right between them.
Note that if you have a subwoofer, then low frequencies will be coming out of the subwoofer rather than the main speakers, so the interference pattern won't match your experience.
You can measure distances on the view by clicking the mouse and dragging.
You can also view the interference pattern between one speaker and a wall by setting the Speaker Separation to twice the distance between the speaker and the wall. Uncheck the Stereo checkbox or set the Phase Difference to zero. Imagine the wall being drawn vertically down the middle of the screen.
It is also interesting to move the speakers right next to each other and then set the phase difference to 180 degrees.
