Can any one tell me the meaning of these lines...............?

It’s about cooking and eating things. You can cook things separate; like meat, potatoes, onions, etc. and then eat them all separate. Or you can cook things like stew, where all the things are swapped around in the juices. Even if all is cooked separate, once it’s on your plate you can then swap things around in the juice (gravy) and eat them that way. What you eat is going to get swapped around in the juices in your stomach, anyway.