In a very simplified way we can say that In the "real" world, bank account balances are only addressed at the close of the day. For example if a bank account holder has a negative (short) position in, say, USD then they will borrow funds overnight to balance their account on each close of day. Similarly, if an account holder has a surplus in their account then they will lend it out overnight. These overnight funds have an interest rate cost to the borrow, and a gain to the lender.
The same applies to our positions with our broker if we wish to retain an open position overnight. Intraday trades are not affected by this as they are closed out before the end day balancing process.
I think this "Wikipedia eplanation is quite specific:
"Trading platforms offer rollovers but the process involves a rollover interest fee which is calculated according to the difference between the interest rates of the traded currencies. If the interest rate on the trader's long position is higher than the rate on the short position, the trader receives the interest. If the interest rate on the trader's short position is higher than the rate on the long position, then the trader pays the interest. For weekends and holidays, the rollover is multiplied by the number of days of rollover."
Being in bit of a holiday mood right now, a rather different definition comes to mind, being:
"The minimum level of commitment required in order to get out of bed in the morning and switch on the computer" - I know, that a bit facetious really, sorry!