An index fund holds the same stocks, in roughly the same weights, as a published market index. The fund manager’s job is to track the index as closely as possible, not to outperform it.
Because there is no expensive research team trying to pick winners, expense ratios are typically lower than actively managed funds. Over long periods, lower costs can make a meaningful difference to net returns.
Index funds still carry market risk — if the index falls, the fund falls. They are a core building block for many goal-based portfolios, not a risk-free product.