An index fund is an investment that holds a whole "basket" of assets to track a market index, instead of trying to beat the market by picking winners. Educational only.
An index (like a broad stock-market index) measures a slice of the market. An index fund simply owns the things in that index, so it rises and falls with the overall market rather than betting on individual stocks.
Buying one broad index fund spreads your money across many companies at once. If one company stumbles, it's a small part of the whole — diversification reduces the risk of any single bet blowing up.
Because index funds don't pay teams to pick stocks, their fees are typically very low. Over decades, small fee differences compound into large amounts — low cost is one of the biggest reasons they're favored.
They're simple, diversified, low-cost, and don't require you to analyze companies. Historically, broad low-cost index funds have been hard for active pickers to beat over the long run.
They still fall in downturns — they track the market, including its bad days. They're a long-term tool, best paired with regular contributions and patience (start investing).