A spawn cube is half of a pair of things that depend on each other to function. In this case, it is the spawn spot. When the cube "launcher" comes out of its dormant state, the famous backwards message sounds, and the cube is launched towards one or more preset targets.
While the targets may be at any elevation, the cubes themselves travel horizontally, due to the limits of the Doom rendering engine. It rotates slowly as it travels, and each face of the cube has a skull imprinted on it.
If the player or a monster is standing in the target zone when the cube impacts, a Telefrag results.
The spawn cube sequence is part of the Final boss illusion, as it is set up to trick the player into thinking the boss's brain is ejecting the cubes.