Swap is the term used to show the interest that is received or paid for keeping a trade open overnight.
Swap rates come from the interest rates of the central banks of the two currencies that you are trading with.
Swap short is when a short position (Sell) is kept open overnight.
Swap Long is when a long position (Buy) is kept open overnight.
This is how you can calculate the swap fee:
Swap Long: One Point x Trade Size x Swap Long in Points
Swap Short: One Point x Trade Size x Swap Short in Points
Here are two examples to show how this calculation works. It will show how you can lose or gain money through swaps.
0.01 x 100 x 1.24 = 2.53 USD
This means that you would receive $2.5272 in swap fees for keeping 1 lot short of XAUUSD past the end of day (5 pm EST).
0.00001 x 100,000 x -4.98 = -4.98 USD
This means you would pay $4.98 in swaps for keeping 1 lot short of USDCAD past the end of day (5 pm EST).
If you are unsure of what the figures in the above equations are, or where they can be found, here is a short explanation on each:
One Point = 0.00001 in EURUSD is the last digit in the price
Trade Size = Lots * Contract Size = 100,000
Swap Long in Points = -4.98 on a Long position on USDCAD
Something to note is that with pairs that close on weekends, there will be a triple swap charged. This is charged on a certain day, usually a Wednesday but in some cases can be another day, to make up for the 2 days that the market is closed.