Key currencies are those that are stable, usually making them an international anchor for other currencies. For instance, one country might use another country’s currency rather than their own for a foreign exchange transaction...a key currency, that is.
It’s common for governments to actually hold onto key currency in their governmental coffers...giving them liquidity in all the places. Foreign governments also might peg their currency to a key currency to increase economic stability (hopefully). It’s also common for big-boy commodities, like oil, to be priced on the international market in key currencies (cough cough...U.S. dollar...cough cough).
Besides the U.S. dollar, which can be found in cash registers in corner stores of Cambodia, other key currencies include the euro, the British pound, the Japanese yen, the Canadian dollar, the Swiss franc, and the Mexican peso.