It's best if I explain rollover interest to you in steps.
First, consider how it works when you deposit money in a bank account. Effectively, you are lending money to the bank, so you can earn interest on your money which the bank pays you. Conversely, if you get a loan from a bank, then you must pay interest to them for the money you borrow.
Rollover interest in the forex market works the same way, except you earn or pay it daily instead of monthly, and it involves two currencies: one that you are long, and one that you are short. For example, if I buy USD/JPY, then I am long US dollars and short Japanese yen. If I sell USD/JPY, then I am short US dollars and long Japanese yen.
I earn interest on the currency I am long, while I pay interest on the currency I am short. That means, for any currency pair, I will earn rollover interest, if I am long the currency in the pair with the higher interest rate of the two. However, if I am short the currency with the higher interest rate, then I will pay rollover interest.
Note that in order to earn or pay rollover interest, your trade must be open at 5pm New York Time on a weekday. That is the cutoff point that divides one 24-hour trading day from the next. Only trades that are held through that cutoff period must be rolled over from one trading day to the next, earning or paying the appropriate day's worth of interest.
The exact amount of rollover interest, I can earn or pay is displayed in the RollS and RollB fields of the Advanced Dealing Rates window. RollS applies if I sold the currency pair. RollB applies if I bought the currency pair.
The information above is taken from one of my trading accounts which is denominated in USD. That means the RollS and RollB values are shown in US dollars. If you have a GBP-denominated trading account, then RollS and RollB will be displayed for you in British pounds.
The amount shown applies for a 10k trade, so if you have a 60k trade simply multiply the amount by 6. For a 2k trade, you can divide the amount show in the Roll columns by 5. That means, if bought 100k AUD/USD and held that position open through 5pm New York Time today, I would earn $2.50 in rollover interest (0.25 USD * 10).
For more information on rollover interest, and how it is affected by weekends and bank holidays, visit the Rollover Calendar on DailyFX.com.