Swap is the interest that accrues when holding a position in a currency pair overnight. All currency trades involve borrowing one currency (the quote) in order to purchase another currency (the base), when holding these positions overnight interest is charged on each currency in the pair. Traders are charged interest on the currency they have borrowed and are paid interest on the currency they have bought. So, depending on whether the quote currency’s interest rate is higher than that of the base currency, a trader may owe interest, or be owed interest when a roll over occurs.
Swap is calculated as follows: Swap = (one pip / exchange rate) * trade size (lot size) * swap value in points. Click on "Trading Calculator" in the menu on the left to perform swap calculations.