Diversification
Spreading your money across many investments so that one bad company or sector does less damage to the whole.
If you own one company's stock and it fails, you can lose everything you put in. If you own thousands of companies through an index fund, one failure is a small dent.
You can diversify across companies, industries, countries and asset types like stocks and bonds. Diversification lowers risk, but it cannot prevent losses when the whole market falls. A broad index fund is one simple way to get it.