You have noticed for sure, that if you leave any transaction open, the next day you will notice the minor change of your account balance even if during this period have not been made any trading activities. The swap is the commission for the transfer of your open position for the next trading day.
But why this happens, when the Forex market is famous for non-stop trading? Despite the status of the free and independent market, the Forex market has certain rules which extend to all his players. According to the exchange canons, at the end of the working day (trading session) have to be made the calculation in real money on all open transactions. However, participants of the Forex OTC market earn on a difference from the purchase and sale of currencies, using electronic terminals and means of communication, that is why this process is physically unavailable to them. Therefore, instead of cash calculation, all transactions are closed and reopened the next trading day. The commission paid for such service is called a swap.
For traders swaps don't represent a big problem, moreover, they figured out how to make money on them. This particular strategy is known as Carry Trade. The main principle consists in purchasing the tool with higher interest rate than quoted at the expense of what it is possible to get profit (a positive swap) in the case of open transaction transfer to the next day.
Nevertheless, do not forget that this strategy has a very high risk due to the unpredictability of the currency market. The risk is that the profit got on swaps for transfer of a transaction cannot block a possible loss from the transaction if the purchase price of a currency pair appears high prices of its sale. Therefore, if you decided to earn on swaps, do it only during those periods when the volatility of the market is minimized.