A corn exchange was a building where farmers and merchants traded cereal grains. They were the forerunner of the modern commodities exchange, which mainly deals in the futures trade of almost any commodity, and are generally at the heart of both liquidity and risk in a growing economy.
In a futures market, a farmer would sell (for example) corn that he has yet to grow to investers who have money now and are prepared to risk that money rather than keep it 'liquid'. The farmer gets a guaranteed price, allowing planning for the year, and the investor hopes that the price of corn will rise, allowing a profit.
Occasionally this speculation can become so widespread that the price of the corn is forced to rise, simply because so much corn has been bought by speculators who hoard it, awaiting a higher price. This can cause a market 'bubble' if allowed to continue without correction, and result in a slump if the bubble 'bursts'.