This page creates "echos" of your touches to the screen.
Your browser captures the touches through the touch events APIs. These events are then used to store where and when you started touching, moved and stopped touching the screen.
This information is in turn used to first display your current interaction, then the same motion is repeated 3 times, each time getting dimmer.
This is currently quite experimental technology, so it might not work perfectly (or at all) in your current browser. Try using multiple fingers, for example. If it doesn't work, try another browser.