A price match is a cruise line agreeing to give your booking a lower fare it is selling for the same sailing. Royal Caribbean's own wording is typical: if you find a lower price for the same cruise, same date and same room type on its website or through its call center, it will match it.
The lower fare almost always has to be the line's own public fare, not a price from another seller. Celebrity requires the same ship, sail date and stateroom category. Carnival requires the same ship, sailing, stateroom category and number of guests, at a rate you are eligible to receive. A cheaper cabin in a different category, or a fare restricted to a group you do not belong to, does not qualify.
What you get from a match depends on the line and on timing. Before final payment it is usually repricing: your fare is lowered, and the difference is refunded or taken off your balance. Later it can become onboard credit or a cabin upgrade. The line decides each request under its own terms.
A price match is only made when you ask for it. The line does not apply lower fares to existing bookings on its own. The rules for each line are on the price protection pages.
