Dow Jones Industrial Average
"It is an index that shows how 30 large, publicly owned companies based in the United States have traded during a standard trading session in the stock market."
The Dow Jones Industrial Average has picked 30 stocks of companies that represent how "industry" in doing in the US. When these stocks are doing good then it usually means that the US "industry" is doing good. When these stocks are doing bad... then it usually means that the US "industry" is doing bad.
An example to help you understand would be studio funds in HSX. If we were to take Disney and buy a radar stock in each of their movies listed at HSX. Each day we could look and see how the average price of all the stocks was doing for the day. We could see each day whether Disney's overall price was rising or falling each day.