A swap in CFD trading means an interest added or withdrawn for holding open position overnight.
Generally, a swap is a type of a financial derivative that is usually used to hedge a certain financial risk (change in interest rates, exchange rates) or to speculate. This is an agreement of two parties on future payments from the underlying asset. A swap is an entity's obligation to a client to buy or sell at a specified time, under specified conditions.
Prior trading CFDs, make sure to understand how CFDs work. Do not underestimate the importance of education. Be aware leverage trading is risky.