Stands for "Standard & Poor's 500", and is a stock index that follows the 500 largest US stocks.
Since the stock index follows such a broad portfolio of stocks, it is often used as a measure of the health of the global stock market. This is because the US stock market is the one with the greatest influence, and the one that affects the global markets the most.
There are two things that determine how large a share of the S&P index a single company should make up. This is a non-publicly divided factor value that is only shared internally by those who set up the equity index, together with the market value of the individual companies.