The exchange rate expresses the national currency's quotation in respect to foreign ones. For example, if one US dollar is worth 10 000 Japanese Yen, then the exchange rate of dollar is 10 000 Yen. If something costs 30 000 Yen, it automatically costs 3 US dollars as a matter of accountancy. Going on with fictitious numbers, a Japan GDP of 8 million Yen would then be worth 800 Dollars.
Thus, the exchange rate is a conversion factor, a multiplier or a ratio, depending on the direction of conversion.
In a slightly different perspective, the exchange rate is a price. If the exchange rate can freely move, the exchange rate may turn out to be the fastest moving price in the economy, bringing together all the foreign goods with it.