The S&P 500 was introduced by Standard & Poor's in 1957 as a market index to track the value of 500 large corporations listed on the New York Stock Exchange and the NASDAQ Composite.
This collection of stocks is intended to represent the overall composition of the economy.
Its exact combination and weightings of various constituencies is adjusted as the economy changes.
Stocks are added and dropped over time as well!
The S&P 500 is considered to be a bellwether and leading indicator for the economy in addition to the default vehicle for passive investors who want exposure to the U.S. economy via index funds.