Mylar is Tough, Flexible, but Prone to Puncture Damage
Mylar food storage bags are made from a stretched film of polyethylene terephthalate metalized with aluminum. In simple terms, mylar is tough stuff. However, metalized plastic is light, flexible, and thin, making it a valuable foil-like substance that conforms to whatever shape the user needs.
Mylar is also durable and robust with high tensile strength. In addition, it is impermeable to light, gasses, and moisture, making it great for food storage. Its other uses include high-atmosphere balloons, solar filters, emergency blankets, and even insulating the lunar module.