Okay, imagine you're eating a burger. The burger has different parts like the meat, the bun, the lettuce, the tomato, and the sauce. Those parts all come together to make the burger taste yummy.
Similarly, when we talk about "the whole beast," we mean all the different parts of an animal that we can use for food. For example, when we eat chicken, we usually only eat the meaty parts like the breast or the legs. But there are other parts of the chicken that we can use too, like the liver, heart, and gizzards.
Using "the whole beast" means that we use as much of the animal as we can, so that we don't waste anything. This is important because it helps us be more mindful of where our food comes from and also reduces waste. It's like making sure you finish your whole burger instead of just eating the meat and throwing away the rest.