The big metal thing you see to the left is a 2mm-thick RF (radio frequency) shield designed to block unwanted radio interference emanating from the Atari 800's circuitry. When electricity flows through a conductor, it generates radio waves that can disrupt commercial television or radio reception. The FCC strictly regulates devices that might interfere with broadcast media, requiring gadget designers to block these undesirable signals with metal shields.
The Atari 800 also contains an 'RF modulator'--a component that converts the computer's video output into a signal that can be transmitted (via a cable) to a TV's antenna jack. Because RF modulators radiate signals that may interfere with TV reception, the FCC mandates that they be heavily shielded.