Garbage plate means: A large serving of mixed fast foods, such as hamburger patties, fries, macaroni salad, etc.. It is informal slang, fine in casual speech but not suited to formal contexts. There is no real cause for parental concern; it is descriptive vocabulary rather than risky behaviour. This is a food-related word and harmless in itself. If your teen uses it, context will usually make the intent clear. A short, curious question about where they heard it is usually all that is needed to know whether to follow up.