it's a pack that comes in the mail 2 weeks before departure if you booked with Disney UK. It contains a guide, discount coupons, and your vouchers for accomodation, tickets, and all.
edit : oh I realise that you might also mean "EarMail" which is stated at time of booking. Those are a couple of emails giving you basic information on your upcoming stay, like weather at the time of year you're going, and so on. Nothing too important anyway.