Potato chip bags are made from a plastic called oriented polypropylene. This product protects against moisture, which is an important part of keeping potato chips fresh and edible.
Food companies put a lot of thought into properly packaging the chips. In addition to the polypropylene, manufacturers use polyethylene and a thin layer of aluminum to keep the chips sealed. To prevent the chips from being crushed during shipping, manufacturers fill the bags with nitrogen gas before sealing them. Some gases would cause chemical reactions with the fat of the chips and affect the taste. However, nitrogen gas doesn't cause chemical reactions so the potato chips taste fresh provided they are eaten by the expiration date.