What is purchasing power parity and what happens when this condition doesn't hold?

What will be an ideal response?

Purchasing power parity means equal value of money. If prices of goods and services are higher in the United States than the (exchange rate adjusted) prices of goods and services in, say, Japan, purchasing power parity does not occur because a unit of currency buys less in the United States than in Japan. The demand for U.S. dollars decreases and the supply of U.S. dollars increases so that the value of the dollar falls against the yen to restore purchasing power parity.