A PET preform (PET hollow body) is the injection molded "preform" of the well-known polyethylene terephthalate (PET) bottle. 
The production of a PET preform goes through several steps. 

First, the preform is heated. Then he comes in a (mostly) three-piece mold. There, a thorn moves into the preform and stretches it to the final length of the bottle. By means of compressed air, the resulting tube is brought to the final shape. This is then cooled (conditioned), the mold opens and ejects the finished "cast" mold. 

If you take a close look at a PET bottle, you will see two long vertical lines as well as an all-round line in the bottom area.