This is a popular trading strategy. Its essence is earn from the differences in interest rates for various national currencies. These rates are announced by the Central banks every month. For example, you have just learnt that in Australia the interest rate is 3% and in Japan it's almost 0%. So you go to your terminal (see) and buy Australian dollars, as due to the news of a higher rate, the currency will rise in value, and sell the yen, sine the minimum bid price of the Japanese currency is lower. So, the most popular currency pairs with the highest rate differences are: AUD / JPY, NZD / JPY, CAD / USD, EUR / USD.