A forex contract expires every two days. Instead of actual delivery of the underlying asset, many traders and investors will ‘rollover’ the contract to extend its trading period. It’s upon this ‘rollover’ that swaps are earned or incurred by the holder of a forex contract. The swap rate is calculated as the interest rate difference between the two nations issuing the currency, and are added to, or subtracted, from the profit and loss for the contract.

A positive swap is where you earn interest from the contract, and a negative swap is where you incur interest on the contract.