A flush door has a basic structure composed of solid blockboard core, vertical stiles, and horizontal rails that create a pre-fixed frame. The blockboard is composed of wooden strips that are placed edge-to-edge and sandwiched between veneers, then bonded under high pressure and temperature using a synthetic resin. So in a simpler term, it is a door that is made of a timber frame covered with ply from both the sides and then the hollow part inside is filled with rectangular blocks of soft wood. Then a decorative finish is given by fixing veneer on the top.
A flush door is so called because it has an entirely smooth surface. If water were to be splashed on its surface, it would simply flow off its surface without accumulating.