What does it mean that diversification is the only 'free lunch' in investing?

It means spreading your money across many investments reduces your risk without forcing you to give up expected return – a rare win-win, which is why it's called the only free lunch in finance. Because different assets don't all move together, owning a broad mix smooths out the ride: when some holdings sag, others may hold up, so the overall portfolio swings less than its individual parts. The simplest way to capture this is a low-cost total-market index fund, which spreads your money across thousands of companies in one purchase, often paired with international stocks and bonds. Concentrating in a few stocks or your own company's shares is the opposite – higher risk for no extra expected reward. You can't diversify away every risk (the whole market can fall together), but you can erase the avoidable risk of any single company sinking you. Diversify first; chase performance never.