Forex, the Largest Market in the World

A daily rollover interest rate is applied to open positions on the market as of 5:00 PM EST. Traders are either paid or charged rollover interest depending on the type of open position.
Traders earn rollover interest when they have open positions, in which the difference in interest between the two currencies in a pair generates sufficient earnings. These positions are called positive rollover positions. In other words, if a trader buys a currency with a current prime interest rate that is sufficiently higher than that of the currency that he sells, the net difference is positive and the trader earns interest as a result. On the other hand, if a trader has an open position in which the currency pair generates negative interest due to the current interest rate situation, the trader will have a negative rollover position and interest is charged. Since all transactions involve the sale and simultaneous purchase of a currency, interest from rollover positions is a normal part of trading. 

The rollover interest rate per lot of trade volume is displayed above the buy or sell price in the Advanced Dealing Rates window of the FX Trading Station. RollS indicates the interest for sell transactions and RollB indicates the rate for buy transactions.