"Glycogen supplies" refer to stores of a type of carbohydrate called glycogen, which are stored in the body and used as a source of energy when needed. Full definition
Remember that the body doesn't have a significant store of carbs; the body's total glycogen supply amounts to about a day's needs. (perfecthealthdiet.com)
It is placed in energy drinks because it is believed to help prevent glycogen depletion by preventing other substances from depleting glycogen supplies in the muscles. (caffeineinformer.com)
Additionally, the more you train, the easier it is to enter ketosis because depleting glycogen supply will be quicker. (onketosis.com)