PET is a type of plastic usually found on things like water bottles and food containers. PET screen films provide the least amount of scratch- and impact-protection, but they're super cheap, light, and thin, and as a result are less visible once applied to your phone.
They also have a decently smooth feel, unlike the more durable but rubbery TPU. PET is also a bit stiff, so it can't go edge-to-edge on phones with curved screens. If you want to go with PET, I recommend Tech Armor(Opens in a new window). And this kind of protector can be devided into many types with different aims and functions like PET Clear Film, PET Clear Anti Blue Light Film, PET Privacy Film and PET Anti Blue Light Privacy Film.