What it means
It depends on the interest rate gap between the two currencies. Many brokers charge three days' swap on Wednesday to cover the weekend.
In practice
Swap matters for trades held overnight. Holding the higher-yielding currency against a lower-yielding one can earn a small daily credit, and the reverse costs a daily charge; brokers add their own markup to both.
Day traders who close before the rollover never pay it. Swing traders check the swap rate for the pair and direction before entering, because over several weeks it can add up to a meaningful part of the result.
What to watch for
Spreads usually widen for a few minutes around the rollover, so stops placed very close to price can be hit then. A trade that depends on collecting swap can lose far more from the price move than it earns in interest.