The daily process of rolling an open position from the current settlement date to the next, triggering a swap credit or debit.
Spot forex trades technically settle in two business days. Brokers automatically roll open positions forward each day at a fixed rollover time (usually 5 pm New York). The cost or benefit of rolling is the swap - reflecting the interest-rate differential between the two currencies.
On Wednesdays, triple swap is applied to account for the weekend. Traders holding positions through rollover must account for swap in their overall trade cost.