The idea behind diversification is captured in the old saying about not putting all your eggs in one basket. By holding a mix of assets — equity, debt, gold and cash — and many holdings within each, you reduce the damage any single failure can do.
If you own one stock and it crashes, your wealth crashes with it. Own 50 stocks across sectors and one crash barely dents the whole. Mutual funds and index funds provide this diversification automatically.
For example, in a year when stocks fall but gold rises, a diversified portfolio holds up better than an all-equity one. Diversification does not guarantee profits, but it is the simplest way to control risk without predicting the future.