Rollover is the interest paid or earned for holding a position overnight. Remember, as Forex currencies are traded in pairs, you are simultaneously buying one currency and selling the other: for example, when you trade EUR/USD, you are buying EUR and selling USD.
If the interest rate of the currency you are buying (in this case EUR) is higher than the interest rate of the currency you are selling (in this case USD), you would earn interest and positive Rollover will be applied to your account. If the opposite is true and the interest rate of the currency you bought is lower than the interest rate of the currency you sold, you would pay interest (negative Rollover will be charged to your account).
Rollover rates are updated and applied daily at 21:00 GMT.
Please note that Rollover applied on Wednesday at 21:00 GMT are multiplied by three (3), in order to compensate for the weekend. This is because Forex spot transactions settle two (2) business days from the date of the actual trade.