This video uses polarized light waves to create its trick. Light travels in waves through an electromagnetic wave, which is a transvers wave, that has both electric and magnetic components. Lamps and candle flames are examples of unpolarized light waves, because the vibrations in the wave travel through more than one medium and travel in different directions. Sun light is also an example of unpolarized light and this is the case in the video. The light coming in from the window on the other side of the card makes the drawing on the side facing the camera visible. Polarized light waves are waves that only vibrate in one medium. You can turn unpolarized light into polarized (Polarization)  by 4 methods (polarization by Transmission, Polarization by Reflection, Polarization by Refraction, Polarization by Scattering.)