Most deals in Forex are done as Spot deals. Spot deals are nearly always due for settlement two business days day later. This is referred to as the "Value date or delivery date. On that date the counterparties take delivery of the currency they have sold or bought. In Spot FX the majority of the time the end of the business day is 21:59 (London time). Any position still open at this time are automatically rolled over to the next business day, which again finishes at 21:59. This is necessary to avoid the actual delivery of the currency. As Spot Forex is predominantly speculative most of the time the traders never wish to actually take delivery of the actual currency. They will instruct the brokerage to always rollover their position. Many of the brokers do this automatically unless you instruct him that you actually want delivery of the currency. Another point noting is that most leveraged accounts are unable to actual deliver of the currency as there is insufficient capital there to cover the transaction.
Remember that if you are trading on margin, you have in effect got a loan from your broker for the amount you are trading. If you had a 1.0 lot position your broker has advanced you the $ 100,000 even though you did not actually have $ 100,000. The broker will normally charge you the interest differential between the two currencies if you rollover your position. This normally only happens if you rolled over the position and not if you open and close the position within the same business day. If the first named currency has an overnight interest rate lower than the second currency then you will pay that interest differential if you bought that currency. If the first named currency has a higher interest rate than the second currency then you will gain the interest differential.
To simplify the above. If you are long (bought) a particular currency and that currency has higher overnight interest rate you will gain. If you are short (sold) the currency with a higher overnight interest rate than you will lose the difference.