Spreading money across many investments so no single loss sinks you.
Diversification reduces risk by spreading money across many companies, sectors, and asset classes, so a poor result in any one place has limited impact. A broad index fund is diversified by design. It does not eliminate market risk, but it removes the avoidable risk of betting too much on a single stock.
Owning one total-market index fund is more diversified than owning five individual tech stocks.