The balance due is the amount still owed on your cruise after the deposit. It is payable by the final payment date shown on your booking, which depends on the line and the length of the sailing. Royal Caribbean and Carnival both publish their schedules, and both set the date further from sailing for longer cruises.
If the balance is not paid on time, the line can treat the booking as cancelled. Royal Caribbean's booking conditions say a guest who has not paid in full and on time will be deemed to wish to cancel, with cancellation charges applied.
The balance matters for price drops because it is where a successful claim usually shows up. Before final payment, most lines handle a lower fare by repricing your booking, which lowers the balance due by the difference. If you have already paid in full, the same claim comes back as a refund on Royal Caribbean and Celebrity, and as onboard credit on Carnival's Early Saver.
That makes the period while a balance is still due the most valuable time to catch a drop. The guide to final payment dates explains what changes once the balance is paid.
