In simple terms: click, move the mouse, and make thereminy sounds.
The WebAudio API is a new web standard under review at the W3C, the governing both of standards for the web. It is supported in the latest versions of the Safari, Firefox, and Chrome web browsers. Like all unapproved standards, there's a good change it will evolve quite a bit before the final version of the spec is ratified.
Stuart has developed a few other Web Audio toys which are posted at his site QuestionPark.com. Among them is a fun little drum machine with classical Linndrum, TR-808, and TR-909 sounds. While not anywhere near as fully functional as actual drum machines, they give you a little idea of just what might become possible very soon with the Web Audio APIs.
For another awesome taste of what Web Audio can do, check out these guitar stompbox pedals on the web. Now imagine you're shopping for pedals and want to hear an example of what your instrument might sound like through them. Just plug in your guitar or theremin to your PC and have a go with the virtual versions! So far, live audio input is only supported in the latest nightly builds of Chrome, but we hope all browsers will support it and the full Web Audio API in the near future. MIDI support is coming too.
Give the Web Audio Electro-Theremin a try at http://questionpark.com/electro-theremin/