What exactly is a Phantom item or Assembly means?

A phantom assembly is used when you want to be able to structure a BOM so it is easy to understand, but don't want to create too many production orders.
Assume an auto Engine. There are hundreds of components. You might structure them as: Engine block and parts, camshaft and parts, and 6 piston assemblies. But you don't want to create 3 production orders, too much hassle. So you want to issue the components for the piston assembly in the same production order as the Engine block. So you create a new material number for the Piston assembly, but you mark it as a phantom assembly. That means that when you create the bom for the Engine assembly, you only have two assemblies, the Engine block and the Camshaft. You add the phantom assembly for the Piston Assembly to the Engine block BOM, saying it requires 8 of the phantom assembly. When the production order is created for the Engine block, the picklist will also include all of the components of the 8 piston assemblies.