A future cruise credit, usually shortened to FCC, is a credit a cruise line issues that can only be spent on a cruise with that line. It is not money returned to your card. It works as a form of payment on a booking you make later.
The most common way to end up with one is cancelling a fare with a non-refundable deposit before final payment. On Carnival's Early Saver fare, for example, Carnival's own help pages say a cancellation before the final payment date returns the deposit as a future cruise credit less a $50 service fee per person, applied to the cruise fare, non-refundable and non-transferable, and it must be used on a booking made within 12 months. Royal Caribbean lets you enter an FCC on its website like any other payment, or apply one to the balance of an existing booking, and reissues any unused remainder as a new credit.
The distinction matters when money is on the table. A refund is yours to spend anywhere. An FCC only has value if you sail with that line again, inside its expiry date and under its currency and transfer rules. Those rules are set by the line and differ between programs, so read them before you accept one in place of a refund.
The future cruise credit guide covers how FCCs work in more detail. For the other forms a cruise line pays out in, see onboard credit and repricing.
