S&P 500
About 500 of the largest US public companies, tracked as one index.
The S&P 500 is a stock market index that follows roughly 500 large US companies. It is the most common way to measure "the market".
How it works
Companies are picked by a committee at S&P Dow Jones Indices and weighted by their market value, so the biggest companies have the most influence on the index.
You cannot buy the index itself. You buy a fund that tracks it, such as an S&P 500 index fund or ETF.
Its history in one line
Over long periods it has risen far more than it has fallen, but it has also had deep drops, including a fall of about 57% from 2007 to 2009 and about 34% in early 2020. Steady buyers kept buying through both.