Each currency has an interest rate attached to it, and the difference in interest rates for each currency pair you hold could result in a debit or credit being applied to your account each night. Rollover (or swap) is a way to increase profits or reduce losses, or being on the wrong side of it can cut into profits or add to losses. Rollover is an important concept in forex trading and one that you should be familiar with if you wish to use more advanced trading strategies.