Swap is an interest fee that is either paid or charged to you at the end of each trading day. In short, this is due to the brokerage 'Rolling Over' the trade from one trading day to the next. When this happens there will be different interest rates based on the pairs you are trading. To reconcile those interest will be charged/credited to your account.
Brokerages do not charge/credit swap over the weekend so they compensate for this by charging both Saturday and Sunday's swap on Wednesday evening. This means that all swap on Wednesday's is tripled. This is actually the case for holiday's as well.
MT4 will do this automatically for traders. It will be added to your P&L and Commissions/fees on the trade.
This is explained more here.