A Best Price Guarantee is a cruise line's promise that if the public fare for your booked sailing falls after you book, you can ask for the lower price. The name is marketing; the substance is a set of rules about which fares compare, what you get back and until when.
Royal Caribbean and Celebrity both use the name, with the same structure. Before final payment the line reprices your booking to the lower fare, and refunds the difference if you have already paid in full. After final payment the remedy becomes a complimentary cabin upgrade, offered when a higher category is priced at or below what you paid and subject to availability. The comparison has to be for the same ship, sail date and cabin type. Royal Caribbean notes the program is not available in every country or for every reservation.
Other lines run the same idea under different names. Carnival calls its version Early Saver price protection. Princess retired its Better Than Best Price Guarantee, so it no longer pays anything on a price drop.
A guarantee does not watch your fare for you. The line only acts when you or your travel agent ask, which is why a drop nobody notices is money the line keeps. The details for each line, with their own published terms, are on the Royal Caribbean price drop policy and Celebrity price drop policy pages.
