Maybe you have heard your parents or news commentators talk about the stock market. Essentially, it is where people can buy stocks, or very small pieces, of big companies like Walmart or Facebook. If the company makes money, the stock-holder gets also gets a small profit. Investing in a stock is like taking a gamble, because the company could earn a lot of money, or it could lose money.
In the years before 1929 the stock market was an extremely popular way for everyday people to earn some extra money. Because of this, the prices of stocks kept getting higher and higher. By 1929, many prices were much higher than the actual values of companies.
Some people started getting skeptical. Prices could not keep going up forever. So they started selling their stocks while they were still at high prices. More and more people started catching on, until everyone was scrambling to sell their stocks at once. There was no one left to buy all these available stocks, so the prices dropped steeply.

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