The Principles of Food Preparation are conceptually basic and, as the phrase suggests, standard across the restaurant and cafeteria industries. These principles set forth the expectations that should be met when one is planning a meal, especially for a large group. They include preparing a menu that is balanced between food groups to ensure nutritional value, proper handling of the raw and cooked food for health reasons, maintenance of a clean kitchen to minimize the risk of food-borne diseases, proper personal hygeine, and proper storage of food both to prevent spoiling and to prevent infestation of bugs and rodents.
When health inspectors visit restaurants and cafeterias, they observe the procedures followed by the kitchen staff very closely, and will indicate when those procedures are below standards. Among those indications of unsafe food preparation are failure of the staff handling the food to wear plastic gloves and hair coverings, improper temperatures in refrigerators and freezers where food is stored, containers unsecured so that roaches or rats can access them, and overall cleanliness. Failure to comply with the Principles of Food Preparation can cost a restaurant business, and can result in monetary fines and desist orders that possibly even close it down.
[Note: the individual who answered this question has considerable experience in the food preparation industry, including volunteering in a school cafeteria preparing meals for children.]