an empty calorie is something that provides calories but no or very little nutrients. the opposite would be i guess a nutrient dense food which is pretty much all good foods
For example, potato chips. While they have 5M calories per .99 cent bag, that is about all you get for your money. Most junk food fits in this category.
soda is a good example of empty calories. its nothing but pure sugar carbs...while a slice of whole grain bread has the same carbs, but has fiber, some vitamins and minerals.