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.
 

Check the live rates below to calculate the positions your entering for positive or negative swaps