The black disks are made from a material that becomes superconducting when submerged in boiling liquid nitrogen (T = 77 kelvin, --196 °C, --321 °F). Superconductors are also super-diamagnetic — magnetic fields can't penetrate them. The little metal bar is a strong magnet. The superconducting disks refuse to allow the magnetic field of the bar to pass through them. They generate their own magnetic field and repel the magnetic bar. This demonstration is called the Meissner effect.