A unit of measure that tells you how much "energy" is in a type of food. Carbohydrates and protein have 4 calories per gram while fat has 9 calories per gram. Some diets also consider the glycemic index of foods, or how fast a food increases the level of glucose in the blood.
How many calories do people need in a day? A very rough rule is to multiply your weight in pounds by 12 - this is probably the minimum calories you need to get by day to day; as an athlete, you will need more - you could burn an extra 800 (or more) calories every hour during a workout. If you want to continue to be able to practice, you need to replace this spent energy. How? By eating!!
Basic guidelines state that in a general diet, approximately 60 % of your daily calories should come from carbohydrates, 15% from protein, and 25% from fats. This will vary from plan to plan and from person to person, and the exact breakdown is up to you.
Remember, you should consult your physician before beginning any type of specific diet to avoid or reduce the chance of medical complications. Most experts also advise athletes to break up their meals into smaller mini-meals throughout the day, as opposed to just a breakfast, lunch and dinner.