Depreciation is when the value of a currency is lowered. The depreciation of the U.S. dollar when compared to the Euro, for example, means that you'll pay more for things in Europe at the current exchange rate.
Depreciation is from the Latin word depretiare, which means to lower in price, with the roots de, meaning "down," and pretium, meaning "price." If something goes down in value, then the amount of the decrease is called depreciation. If, for example, you buy a car for $10,000, but a year later it is only worth $8,000 due to wear and tear, then the depreciation on the car is $2,000.