Hi
It means you can check in all your luggage and get your boarding passes at a counter set up in downtown disney on the morning of your departure. This frees you up to do whatever for the rest of your last day, I think you have to get to the airport a couple of hours prior to departure, but no queues!
Also, if you do need to check in any further bags afterwards they allow you to do it when you arrive at the airport.
Its one of the main reasons I travel with Virgin airlines, when I can.
Have a great hol! Jo x