A reserve currency is the one used as a means of payment in international transactions due to its confidence. In other words, investors expect their value to be maintained over time. Currently, the dollar is the currency that mainly meets these characteristics.
An important part of the international reserves of each country is denominated in some reserve currency. The bodies responsible for pursuing this objective are the central banks.
Reserve currencies also serve to set the prices of commodities such as oil, gold, and other raw materials. The same happens with financial costs and international transport.