By typing on your keyboard you will generate a rectangle wave. The wavelength is depending on the ASCII code of the letter you type. This sound is picked up by a processing sketch. This is visualizing the sound. Typing more than one letter at once, you create a very high pitched tone and unexpected visuals.
The visuals are depending on some other factors than the letters you type, it's also depending on the quality of the sound system you have, the volume of the sound and the microphone you use to pick up the sound.