There are 2 types of onboard bookings. There is a "placeholder" booking that you can book for $250. Use this type if you are unsure of the cruise you want to take. Once you decide you can covert this to an actual booking. You will receive a 10% discount, if applicable (there are block out dates) and you will be required to pay any additional deposit amount.
The other is when you have already picked out a cruise date. You can make the booking onboard, pay your reduced deposit, get your discount, (again, if applicable).
With both of these onboard booking types you will be required to sail within 48 months of booking to keep your onboard booking perks.