In the forex (FX) market, rollover is the process of extending the settlement date of an open position. In most currency trades, a trader is required to take delivery of the currency two days after the transaction date. However, by rolling over the position - simultaneously closing the existing position at the daily close rate and re-entering at the new opening rate the next trading day - the trader artificially extends the settlement period by one day.
Often referred to as tomorrow next, rollover is useful in FX because many traders have no intention of taking delivery of the currency they buy - rather, they want to profit from changes in the exchange rates. Since every forex trade is transacted by borrowing one country's currency to buy another, receiving and paying interest is a regular occurrence. At the close of every trading day, a trader who took a long position in a high yielding currency relative to the currency that he or she borrowed will receive an amount of interest in his or her account. Conversely, a trader will need to pay interest if the currency he or she borrowed has a higher interest rate relative to the currency that he or she purchased. Traders who do not want to collect or pay interest should close out of their positions by 5pm ET.
Note that the interest that is received or paid by a currency trader in the course of these forex trades is regarded by the IRS as ordinary interest income or expense. For tax purposes, the currency trader should keep track of interest received or paid, separate from regular trading gains and losses.
To learn more, see A Primer on the Forex Market, Getting Started In Forex and Common Questions About Currency Trading.