What is the purchasing power parity theory of exchange

Question
What is the “purchasing power parity” theory of exchange rates? If the price of a representative bundle of tradable goods is currently $5000 in the U.S. and 550000 yen in Japan, is the $ undervalued or overvalued when the exchange rate is 90 yen per $?
5b) Why don’t actual exchange rates move to purchasing power parity levels in the short run?