An index fund is a mutual fund that mirrors a specific index, such as the S&P 500. In this case, the S&P 500 is called the index funds benchmark. Rather than buying every stock in the S&P 500, an investor can simply purchase an S&P 500 index fund, since index funds contain a similar composition of stocks found in the benchmark index.
Because index funds are passive investments, the investor only needs to buy the fund. They don’t have to worry about managing any of the investments within the fund to match the benchmark. The passive nature of index funds also means their expense ratios are fairly low. Index funds are available to individual brokerage accounts and many retirement accounts.