The CURRENCYRATE function in Salesforce returns the conversion rate from your corporate currency to the currency you name with its ISO code, for example CURRENCYRATE("EUR"). Multiply an amount in the corporate currency by it to convert the amount, and expect 1.0 when the code is invalid or the org doesn’t use multiple currencies.
I tested the function in a Salesforce Developer Edition org that has a single currency (INR), which shows a case most articles skip, and I explain how the result changes once multi-currency is on. You will also find the correct way to convert in both directions and the dated exchange rate limitation.