The design is based on a touch screen technology known as FTIR or Frustrated Total Internal Reflection. There are basically infrared LEDs placed at the edge of the glass. In this case, they are hidden inside holes where the glass is mounted to the metal base. Because of the low angle at which the light impacts the internal glass surface, it is bounced around inside the glass, 
much like looking through a tube to see the reflections on the inside walls.
When the glass is touched, it frustrates the reflection and the IR light is diffused downward, out of the glass, allowing the camera to see it. Software then determines the location and sends the appropriate information to the micro.
For more information on this concept click Kickstarter through which the design was funded for interface and use as a computer keyboard.
