Macroeconomics In a Paragraph

Brad DeLong sums it up well:

In normal times, when one investor wants more liquidity or safety, another will be willing to take on duration and risk, and they will simply swap portfolios at current market prices. But in abnormal times, they cannot: The semiconductor fabs are long-run, durable, risky assets that cannot practically be liquidated. And so when the everybodies all decide that they want liquidity and safety -- well, the economy cannot magically liquidate the fixed capital stock at a reasonable price. And to liquidate at falling prices creates mass unemployment.

(HT Arnold Kling)