Rollover is the process of extending the settlement date of an open position (i.e. date by which an executed trade must be settled). The Online Trading market allows two business days for settling all spot trades, which implies the physical delivery of currencies.
In margin trading, however, there is no physical delivery, so all open positions must be closed daily at end-of-day (22:00 GMT) and re-opened on the following trading day. This pushes out the settlement by one more trading day. This strategy is called rollover.
Rollover is agreed on through a swap contract which comes at a cost or gain for traders. Veracity Markets does not close and re-open positions but debits/credits trading accounts for positions held open overnight, depending on the current interest rates (LIBOR/LIBID with added mark-up).