The S&P 500 is an index, or way of keeping track of, a group of 500 large companies in the United States. It includes companies like Apple, Coca-Cola, Disney, Microsoft, and Walmart. Every day, the prices of the stocks of all of these companies go up and down. The S&P 500 is like a scorecard that tells us how all of these 500 stocks are doing on a particular day. If the score is higher, that means that the stocks of those large companies are doing well, and if the score is lower, that means that the stocks of those large companies are not doing so good.