This is "exchange rate parity", which is one of the exchange rate determination theories. Generally speaking, the forward premium rate of the exchange rate is equal to the difference between the interest rates of the two countries. If the domestic interest rate is higher than the foreign interest rate, the local currency will depreciate in the long run, and vice versa.

Because, driven by arbitrage investment, the financial market always develops in a balanced direction, and the change of exchange rate offsets the difference of interest rates between the two countries, thus making the market balanced.