An index is a defined group of securities used to represent a market, sector, or other slice of the investing world. It is a way to measure and describe that group; it is not itself a security an investor buys directly. Funds may be designed to track an index, but the fund and the index are different things.
Understanding what is being measured—and how a fund is structured—can make market conversations easier to follow. This is general education, not a recommendation to invest.