This type of ship is an LSD-a Landing Ship Dock. The entire aft part of the ship consists of a well deck that can be opened to sea by lowering a stern gate and flooding ballast tanks, lowering the ship in the water. Think of the well deck as a big floating dry dock. When it is flooded landing craft of different types can operated. This ship is designed specifically around the LCAC (Landing craft air cushion) but it can also launch LCU (landing craft utilty), LCM (landing craft mechinized) or AAV (amphibious assalt vehicles).
An LCAC preparing to enter the well deck of an LPD.