The premise couldn't be simpler: each letter on your keyboard corresponds to a musical note. What you type is displayed in the panel on the right with a nice gradient. When the app detects you've stopped typing, a little white indicator appears above the letters and bounces along each one, playing the corresponding note as it does so. The gradient indicates which one of 6 "moods" is currently active. Changing the mood changes the speed and timbre of each musical note.
The app is elegantly designed and all the notes have been carefully chosen so that whatever you type, you'll always end up with a pleasant sounding tune at the end. We've also managed to recreate some classics: Twinkle Twinkle Little Star is "eettrrt hhaadde ", for example.
Jono has written a detailed post about the process that went into creating the app, which you can see here. There are versions for iPhone and Android, and even an embed code, so you can embed it in your own website, like I've done below.