Schmaltz basically just means rendered fat, but it's generally used to specifically describe chicken fat with onions. The idea is to simultanously render it and infuse it with onion flavor. The leftover crispy chicken skin and onions are called gribenes. Since we can't all afford to have meat every day, and relatively few humans ever could, supplementing our diet with meat fat makes good nutritious sense. Animal fat ensures that whatever other protein we eat will get used efficiently by the body. So even if you have to have beans and rice for a month, you'll still get your barest needs met if those beans have a bit of schmaltz.