Michael Pollan summarized the entire body of reliable empirical research on diet as:
Eat food. Not too much. Mostly plants.
(“Eat food” meaning to avoid highly processed “edible food-like substances”.)
I was always told that you should do most of your shopping along the periphery of the store, and avoid the aisles as much as possible.
Which I mostly do, only usually venturing into the aisles for spices, oyster crackers, taco shells and some canned goods(mainly beans and tomatoes).
I know exactly how much sugar to add to half a gallon of water to make 1800 cal - for a while before we figured out my digestive issues were my colon being closed down to a trickle by a 15 cm long, full depth, full circumference stage 3 tumor sugar water was about all i could manage to keep down, flavors and odors were my downfall most days.
[2 1/3 cup if you must know =) - roughly the equivalent to swilling sugar sweetened sodas]
Semi-relevant useless trivia question: Would you longer live (survive) on daily 100 g baked potato diet or 100 g potato vodka diet? Any volunteers? 
I don’t see the confusion.
A balanced diet contains a variety of foods, supplying carbs, ft, and protein; plus assorted minerals and vitamins, fiber, and everything else that makes for a healthy diet in balanced amounts.
A diet consisting of basically, say, just a sugar drink or high-fat carbs can provide the actual “necessary calories” that the body needs, but without those other contents needed for the body to live, your health may deteriorate. We don;t actually see this result because, as some point out, we have enough variety even in a bad modern diet to avoid the worst consequences.
There is plenty of evidence that a diet lacking in certain things can create all sorts of problems - not just deficiencies like rickets, scurvy, etc. but also diabetes, coronary disease an other effects from a diet without balance. A foodstuff which provides a lot of calories in one form - usually fat or carbs - but not much in the way of additional minerals, vitamins, etc. is considered “empty calories”. One of the detrimental issues is that someone who fills up with these “bad” foods may not be motivated to vary their diet to take in other foods that can supply what is missing. Of course, if the overall diet is good and the purpose of these additional “empty calories” is to load up the body then it can be ok - much as allegedly marathon runners will load up on carbs leading up to a run, so they have extra energy. Plus as pointed out above about french fries - “bad” is relative, unless perhaps we’re discussing sugar water. And if the “bad food” is only a small portion of the overall diet, who cares? Still eating enough healthy stuff.
And… there’s plenty of contradictory claims on the internet. As Abraham Lincoln said, “never trust everything you read on the internet.”
I had always heard it as high-carb, high-fat, low-protein food meaning a lot of calories without much satiation.
I think that’s the key point. Even if you eat a terrible diet you’re very unlikely to be deficient in critical vitamins and minerals, as mentioned above. Those “empty” calories give you basically only the calories, and not the other things that make you feel full. It takes something like five or six apples to get one glass of apple juice. You don’t get the fiber from the apples that makes you feel full and which takes energy for your body to process. You get all this sugar that doesn’t actually satisfy you, and your body tries to store it all as fat. Snack foods ramp this up even worse.
Eh, my diet is fairly good. And I eat until I am full. And doing that I haven’t gained any weight. My diet consists of “I should be able to tell what plant or animal gave up its life to feed me” for the majority of what I eat.
Except I gained a huge amount of weight when I was pregnant. I think I must have had some minor metabolic imbalance – I was starving ALL the time, I didn’t have the energy to function if I didn’t keep eating, my blood sugar was too low in the diabetes test, and I gave birth to tiny babies with no body fat.
And after I gave birth, I’ve never lost that weight. I haven’t GAINED weight, either, at least, not to speak of. (I gained a little when I took up a hobby that encouraged after-dinner social snacking, and lost all that weight during the pandemic, without trying to do so.) But I think I’d have to restrict my diet and go hungry to actually lose weight. And I’m not sure I could keep it off without continuing to do so.
Of course, for much of our evolutionary history, getting enough Calories was often difficult, enough so to be a bigger problem than not getting enough of other nutrients (and certainly a bigger problem than the problems that come with too many Calories). Which is why we’ve evolved to like high-Calorie foods like sugars and fats, so we’d be motivated to seek them out when we could, and thus get what we needed.