What is a PET preform mould?
I'm going to guess that the correct term you are asking about is a ‘PET preform mould’ not ‘preformed’, just to help clarify things.
A PET preform is a test tube shaped piece of plastic that is made by a process called injection moulding, using a plastic called PET.
Meanwhile a mould is a shaped hollow piece of metal, a tool, that is used to control the shape of the item being moulded. The mould is what the molten plastic fills under pressure and then cools and goes hard retaining the shape described by the mould.
So a PET preform mould is a mould (tool) used to make PET preforms.
Subsequently, and at a time convenient to a soda, or suchlike, bottling company, that preform will be reheated, and compressed air forced into it, thereby expanding the, soon to be former, preform to become the full shape of a bottle. (Or container). That process is called Stretch Blow Moulding, (not to be confused with Extrusion Blow Moulding). Sometimes the two moulding processes, the injection and the stretch blow, are linked into a dual machine synchronizing the two processes, that is called Injection Stretch Blow Moulding.
I guess it's obvious that the thing is called a preform because it's not yet a fully grown bottle. It's much more convenient to transport small test tubes than full sized bottles.