A Baggie is a small clear plastic bag, the kind you put a sandwich, fruit, or leftover snacks in. In American English this is ordinary kitchen language, and it often keeps a capital B because Baggies began as a brand name — though actually many people now say baggie for any little plastic food bag, not only that brand. They are thin, see-through, and sometimes have a zip to close them. In Britain people are more likely to say sandwich bag or freezer bag. It is informal, lunchbox-and-kitchen talk, not a word you need in a formal essay.