Ever trim your bonless skinless chicken breast and have meat left between the fatty section and the breast? Or trim a steak or roast and have some meat left behind with the fat, or roast a chicken and have wings/drums that no one wants? Or the skin and bones from a roast that you do not want to use for stock that day? Tomato peels from canning salsa? Stems from bunches of herbs? Or even just the liquids from steaming veggies that hold those precious vitamins and nutrients that cooked out?
I keep "stock scraps" together in my freezer, each in its own labeled bag: poultry, beef, and vegetable.
When I trim meat, I add the fat if it still has meat attached. Same with the veggie liquid, scraps from onions/carrots/celery/etc, and other items listed above.
When I am making stock, I add the scrap bag to boost up the stock.
Next to nothing is wasted in our house!