A rollover interest fee is calculated based on the difference between the two interest rates of the traded currencies. If the rollover rate is positive, it's a gain for the investor. If the rollover rate is negative, it's a cost for the investor.
A rollover fee, also known as “swap”, is charged when you keep a position open overnight.
A forex swap is the interest rate differential between the two currencies of the pair you are trading.
It is calculated according to whether your position is long or short.