In world financial circles, "Dr. Copper" is a nickname jocularly applied to the idea that the price of copper is an outstanding leading indicator of the health of the worldwide economy. Because of copper's widespread use and variety of applications throughout the economy, especially its use in any kind of electronics or power generation and transmission, a fall in demand for copper (and thus a dive in its price) is viewed as a signal that the economy is headed for a downturn, whereas a rise in demand for copper (and an increase in its price), is seen as a sign that the economy is gearing up for expansion.
Thus in describing these price changes and what they might mean for the future of the economy as a whole, the copper price is often personified as a "doctor" who "diagnoses" the health of the economy.