A Swap or Roll-Over is the interest paid or received when maintaining an open position.
All currencies yield an interest rate when a customer makes a deposit to a financial institution. Since the Forex market offers the ability to trade many currency pairs through crossed pairs. When you buy a currency in the Forex market, you automatically sell the crossing currency and vice versa. If the interest rate from the currency you buy is higher from the one you sell, then the swap will be positive and you will make money just from maintaining that position open for longer.
If the interest rate from the currency you buy is lower, than the swap will be negative and you will pay a cost to maintain that position open. This cost is charged for keeping the position open overnight, and it is charged at 5PM Eastern Time.
When you buy the EUR/USD pair, you are going long on EUR and short on USD. If the interest rate attached to EUR is 5% and the interest rate on USD is 3%, then the interest rate differential is positive and you will earn a 2% annual yield just by maintaining the positions open.
If the interest rate on EUR is 2% and the interest on USD is 3%, then the negative interest rate of 1% would be charged annually in order to maintain your position open.