In Salesforce, there is a field type "currency", for example, Amount in Opportunity, and we can also create custom fields with the data type of currency.
If you enable multi-currency in your Salesforce instance, and if the record currency is not in your user currency, you will see your user currency show in parenthesis after the amount.
In the report, you will find the (converted) fields for each currency field. You can add it to the report like another normal field. But, you cannot add the converted field as a report filter (as of now).
Solution: use original currency as a filter and add currency code. Example: USD 300, SGD 500, JPY 10,000 and etc.