A swap is a financing cost or credit designed to account for the interest-rate difference between the two currencies in a position held overnight, applied at the daily rollover.

Swaps matter to evaluation traders in two ways. First, as a cost: negative swap on a held position is deducted from equity, which means a position can drift toward a risk limit overnight without the price moving. Second, as a rules question: rollover typically coincides with the daily reset and a spread-widening window, so positions held through it face three simultaneous effects. Triple-swap days (usually Wednesday in forex) multiply the financing effect to cover the weekend.