When people refer to the stock market being up or down, they’re generally referring to one of the major market indexes.
A market index tracks the performance of a group of stocks, which either represents the market as a whole or a specific sector of the market, like technology or retail companies. You’re likely to hear most about the S&P 500, the Nasdaq composite and the Dow Jones Industrial Average; they are often used as proxies for the performance of the overall market.
Investors use indexes to benchmark the performance of their own portfolios and, in some cases, to inform their stock trading decisions. You can also invest in an entire index through index funds and exchange-traded funds, or ETFs, which track a specific index or sector of the market.