Future cruise credits (FCCs) are like store credits, or gift certificates. Cruise lines issue them in a variety of cases - including certain cancellation circumstances, such as voluntary bumping, or other unexpected issues that cause a cruise line to cancel a sailing guests were expecting to be on. You can use FCCs to book another cruise in the future with the same cruise line. Generally FCCs are for cruise fare only, and not for shore excursions, taxes, or other non-fare items. Different cruise lines have different policies around how these credits can be used - such as the dates during which they're valid, whether they must be used all at once/on a single sailing, etc.
Because these are not "vouchers" for a cruise, this means if you're issued a credit, you can use it to book any other cruise with that cruise line (inside the restrictions they might have) - you don't have to take the same itinerary or sail on the same ship you were originally booked on. Depending on what you book, your next cruise might cost more or less than the one you'd booked previously. If it costs more, you'll be responsible for paying the balance.
A number of people received future cruise credits as a result of the mass-cancellations during the 2020 coronavirus outbreak. You can read more (and watch a video) on FCCs as they pertain to coronavirus cancellations at this link.