A swap, or rollover, is an interest difference settled on a daily basis.
This is because currency trading involves borrowing one currency to buy another. Swaps can be positive or negative, since interest is earned on the currency that’s bought, and paid on the one that's borrowed.
At midnight each day the difference between the two is settled and your account will either be credited or debited. Swaps also apply to instruments other than FX, since they are priced in a base currency, which itself has an interest rate.