Calories are a unit of energy. In NURVV Run, Calories refer to the amount of energy that is expended when running and the number of calories burned is dependant on the duration and nature of the run.
Typically, the energy expenditure of running is approximately 800-1000 calories per hour (dependent on an individual's body mass and running speed). The main 'fuel sources' are provided through carbohydrate (glucose) and fat metabolism. Protein ingestion is also crucial, especially post-exercise to aid muscle protein synthesis and repair.
Maintaining high energy levels helps to improve running performance regardless of running abilities/experience of a runner. Generally, all runners should benefit from some form of pre and post-run nutrition even on runs of short distance or low intensity. Intake of additional nutrition, however, should be adjusted to the intensity of the run, especially when covering longer distances. Rehydration also is essential - runners should try to ensure they replace water and electrolytes lost during running.
Some approaches to nutrition for running can be found in Recovery for Runners/Nutrition.