Carbs are basically sugars, but they are pretty much everything your body runs on. Carbs are what gives your body energy. The reason why those girls say that is because they don't exercise enough to burn all that energy.
Carbs (short for Carbohydrates) are a source of energy for humans. If someone is active, they can use all of these carbs, any unused carbs will turn to fat (which is a storage method for energy) if the person doesn't use them.
Excessive carbs lead to fat and weight gain. Pizza, pasta, donuts, pie, bread, cake, all the fun stuff. Somebody watching carbs will stay away from these foods, moderation combined with exercise is key.