A placeholder is a booking for a future DCL cruise with the specifics yet to be determined. For a $250 deposit, you will get to book a future cruise at 10% off (as long as it is non-concierge) and enjoy an onboard credit of $100 (cruises of 2-6 nights) or $200 (cruises 7 nights or longer). You will also pay a reduced deposit for cruises 7 nights or longer. You just have to sail within 24 months of the date that you booked the placeholder.
To book the placeholder, go to the Disney Vacation Planning room on Deck 4 by Shutters. You can either pick up a form to fill out and drop it off or talk to a CM there to do the booking.
And I agree with the PP, totally worth it!!