The place where you can trade shares in companies like Teledyne Paper, or Anacott Steel, or Weyland-Utani, or Tyrell Corp., or Wayne Enterprises, or Paper Street Soap Co., or Dunder Mifflin Paper Co., or Willy Wonka Chocolate Co.
Actually, it's a kind of fake stock market, where people can buy "shares" in actors and movies. The market uses a fake currency, called Hollywood Dollars, which you use to buy virtual shares. These move up and down in value based on the supply/demand for them.
The HSX is largely used as a prediction mechanism for things like box office returns and Oscar nominees.