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 USD, Euro, and CAD, respectively. Now, if you set USD as the base currency, the other two currencies should be converted to USD.
For example, 1 USD equals 0.96 Euro and 1 USD equals 1.42 CAD. Now, if you set the USD currency to base currency, the Euro currency rate to 0.96, and the CAD rate to 1.42 from the currency setting, the conversion will happen promptly.