In simple terms a swap rate is a holding cost. Specifically, a swap rate is a rolling interest rate, which can either be a debit or a credit to any trades held overnight. Financial institutions calculate swap rates based on market conditions such as volatility and risk premiums, and they fluctuate daily.
As swaps are calculated daily, some markets will have a 'triple swap day' to account for the weekend when markets are closed. Typically such a day is on a Wednesday or a Friday, depending on the market.