Entire aisles in the grocery store become irrelevant to you. A cookie is a high-density food. It contains lots of sugar and fat, so it is lots of calories in a small package. A banana, on the other hand, is low-density. It takes many bites to eat a banana, but you take in only 100 calories.
Apples contain few calories for their size.
Or a dozen SnackWell's cookies. That's 600 calories. But it's the same number of calories. Look for low-density foods like bananas that fill you up without giving you that many calories. Nuts, most meats, candy, cookies, crackers, potato chips, fried anything, cola, beer, and so on are all high-density and should be avoided.