A swap, or rollover, is the interest adjustment applied when you hold a position past the daily market close. It comes from the interest-rate difference between the two currencies, and can be a small charge or a small credit depending on the trade's direction. Because of weekend settlement, most brokers apply a triple swap on Wednesdays.
Hold a EUR/USD position overnight and a small swap is added or subtracted at rollover. Keep it open over Wednesday and that swap is charged three times to cover the weekend.
This definition is for educational purposes only and is not financial advice. Trading involves risk.