One of the key areas of a nutrition label is where "Serving Size" and "Servings Per Container" are listed. If you decide to start tracking your daily calorie intake, you will look at this information in conjunction with the number of calories. Only with the serving size can you accurately assess the number of calories you consume when you eat a food or drink a beverage.
Here is an example: The label on your bag of chips says they provide 160 calories per serving. Unless you take note of the fact that 14 chips constitutes a serving, you might assume the amount of chips you can fit into a bowl is a serving. That's probably a lot more than 14 chips, and therefore a lot more than 160 calories.