Imagine that you are trading the EUR/NZD.
The interest rate on the Euro is 0.05%; the interest rate on the New Zealand Dollar is 3.50%.
Note that the Euro is the main currency and that its interest rate is lower than that of the New Zealand Dollar.
So, if a Trader buys the Euro, he will be charged the rollover; and if he sells the Euro, he will receive the rollover fee.