MylarŪ is a trademark of DuPont. It is a polyester film; sometimes generically called "PET" film. The nomenclature is "DMT Ethylene Glycol Dimethyl Terephalate"; PET is an acronym for "polyethylene terephalate"...
Anyway, it's polyester film - remarkable stuff. If it's "MylarŪ" then DuPont made it. Do a search on polyester film, PET, Mylar - awesome stuff. Frequently "metalized" - a thin layer of aluminum, like on "space blankets", which have a layer of metalized mylar bonded to a layer of polyethelene "fabric", I think (the stuff cheap tarps are made from). I'm sure you get the idea, anyway - oh, another common varient: those gaudy metallic helium filled ballons are made from mylar.
And if you store grains and other foodstuffs in HDPE pails, you probably use mylar bags inside the pails...
I have no idea what the "mylar thread" is - I would imagine shiny flat stuff used to gussy up hand tied fishing flys, but that's just a guess.
Regards,
Tom