In the spot forex market, trades must be settled within two business days. For example, if a trader sells a certain number of currency units on Wednesday, he or she must deliver an equivalent number of units on Friday. But currency trading systems may allow for a "rollover", with which open positions can be swapped forward to the next settlement date (giving an extension of two additional business days). The interest rate for such a swap is predetermined, and, in fact, these swaps are actually financial instruments that can also be traded on the currency market.
In any spot rollover transaction the difference between the interest rates of the base and counter currencies is reflected as an overnight loan. If the trader holds a long position in the currency with the higher interest rate, he or she would gain on the spot rollover. The amount of such a gain would fluctuate day-to-day according to the precise interest-rate differential between the base and the counter currency. Such rollover rates are quoted in dollars and are shown in the interest column of the forex trading system. Rollovers, however, will not affect traders who never hold a position overnight since the rollover is exclusively a day-to-day phenomenon.