Calorie density is the total calories per gram of food. It's a representation of how many calories a food has packed in it for a given size. Some foods are small in size but have a lot of calories packed in them (Eg- Almond)
Others would have a large size but still pack fewer calories (Eg- watermelon) Calorie density is an important factor metric to show how fast once fills up their daily calorie allowance through the day
For people on a fat loss journey, monitoring calorie density of foods is a good way to tackle hunger and eat to stay full for long