Disney Express is a free service that you get when you book your
Disneyland hotel & ticket package with transport. When you get to the Marne-la-Vallée station, on the first floor (i think) is a bag drop off, here you can drop off your bags and head straight to the parks!
When you've finished at the parks and go to your hotel, your bags will be waiting for you for collection near the hotel reception!
I think it also gives you the option of checking into your hotel before you board the Eurostar at St Pancras/Ashford, but I'm not sure... does anyone else know about this?