Answer: The currency rate is the value of a currency in terms of another currency. Suppose, you have three accounts A, B, and C for your company, and each account has currencies USD, Euro, CAD respectively. Now, if you set USD as a default currency, the other two currencies should be converted in terms of USD.
For example, 1 USD equals 0.86 Euro & 1 USD equals 1.34 CAD. Now, if you set the USD currency rate to 1, Euro currency rate to 0.86, and CAD rate to 1.34 from the currency setting, the conversion will happen promptly.