AD is a trade within the Royal Logistics Corps (RLC). The role is straightforward and details are on the webpage below;
Role Finder - British Army Website
You prepare provisions, equipment and vehicles for delivery by planes and helicopters. Once safely loaded, you accompany them into the air and drop them by parachute onto the combat zone. You work as part of a despatch crew, and together you learn to operate, drive and maintain a wide variety of vehicles, including cargo handling equipment.
I'm sure some kindly ALM will tell you how the Aircrew/AD roles interact when airborne.