What is Swap or Rollover?
Swap / Rollover / Overnight Charge - the fee for an open position and stay over night. It can be either positive (credited to your account) or negative (debited from your account), depending on the difference between the interest rates of the countries, with the currency in question is a currency pair that is used in a transaction.
As a brief explanation: If you open and close positions before 22:00 GMT, the broker will provide leverage for free. If you let a post pass at 22:00 GMT, then the broker credits or debits Swap / Rollover / Overnight Charge to / from your trading account.
Europa bank interest rates (ECB) is 4.25% and the interest rate the U.S. bank (FED) by 3.5%. Assume you open a short position on EUR / USD with 1.0 lot. Then you have to sell € 100,000, borrowed at 4.25% per year. When you sell EUR, you buy the USD, which you will earn interest at 3.5% per year. As a result, your cost of (4.25 to 3.5)% per year or if the EUR / USD is 1.2500, then it will amount to $ 937.5 per year (or approximately $ 2.57 per day).
That is, $ 2.57 per lot will be debited from your account every day, if you have a short position (sell position) on EUR / USD. If you have a long position (buy position) on EUR / USD, then $ 2.57 will be credited to your account every day.
In practice, the amount debited will be slightly greater than $ 2.57, and the amount credited will be slightly less than $ 2.57. Difference is the commission broker to rollover / stay (see Contract Specification).
Note: Positions stay, which was conducted on Wednesday to Thursday, will be charged three times as much storage. This is because the value / outcome of the transaction is usually there on Saturdays, but because the banks are closed, then the result of a transaction can only be processed on Monday, so the storage from Wednesday to Thursday will be debited / credited threefold.