In the Chicago Merchantile Exchange Building, crazily fast-paced transactions of money and stocks occur all day. One small decision made by a single trader can change the economy in a country across the globe in seconds.
Some of the acts of buying and selling and the exchanges of money and ideas occur on computer systems but most are dictated by various gestures and signs made by actual humans crowded together in distinctly colored jackets, frantically waving their hands and fingers on the trading floor.
This is known as 'the pit' and many consider it a great thing to be working there among the seemingly chaotic mayhem that is actually a system of carefully timed choices made with amazingly high stakes I don't even pretend to understand.

The pit at the Chicago Merc is featured in a scene in Ferris Bueller's Day Off, you may remember it. While Ferris is proposing to Simone, Cameron is busy watching the traders and frantically mimicking their gestures. He would like to work in the pit.