A swap is the interest rate adjustment applied to positions held overnight in your trading account. It can either be a cost or a credit, depending on the asset you are trading and the direction of your trade (buy or sell).
Swap fees are calculated replicating real market conditions, based on the difference in interest rates between the two currencies in a forex pair or other asset classes and are applied at the end of the trading day.
If you hold a position overnight, you may either pay or receive a swap.
If you close your position before the end of the trading day, no swap will be applied.
You can always check the specific swap rates on your trading platform or through your broker, as the rates and policies may vary depending on the broker you're using.