Many brokers hold credits and debits interest on open positions. It's calculated on overnight positions or in real time.
Interests are calculated with the difference between interest rate of the two currencies of your position. For example if you trade Euro Dollar, the concerned central banks are ECB (European Central Bank) and FED (Federal Reserve). If the ECB's rate is 2.25% and FED's rate is 4.25% the difference is -2%. A buy position on Euro Dollar will generate a debit of 2% interest on your account, a selling position will generate a credit of 2% interest.
The calculator would give you the interest rate which you'll pay or will be paid with your trading positions. The interest rate is calculated with the duration and the size of your position.