Simply put, energy density is the number of calories (energy) in a given amount (volume) of food. For weight control, your best bets are healthy foods that provide low energy but high volume. By choosing these low-calorie foods, you can eat more and feel fuller on fewer calories. Foods such as fruits and vegetables tend to be low in energy density — they're low-calorie foods.
So what about raisins? They actually have a high energy density — they pack a lot of calories into a small package. For example, 1/4 cup of raisins has about 100 calories.