In Foreign exchange trading a "Spot" basis means that all trades settle two business days from inception, as per market convention. The settlement date is referred to as the value date. There is no physical delivery of currencies hence, all positions left open and will be rolled over to a new Value Date.
If you have a long position (bought) and the first currency in the currency pair has a higher overnight interest rate than the second currency, then you receive a gain. If you have a long position (bought) and the first currency in the currency pair has a lower overnight interest rate than the second currency, then you lose the difference.
If you have a short position (sold) and the first currency in the currency pair has a higher overnight interest rate than the second currency, then you lose the difference. If you have a short position (sold) and the first currency in the currency pair has a lower overnight interest rate than the second currency, then you receive a gain.
The act of rolling the currency pair over is known as tom.next, which stands for tomorrow and the next day.