Carbohydrate metabolism in general has 2 phases: using sugar to make ATP, or alternatively using ATP to make glucose.
Using sugar to make ATP:
--when we eat, carbohydrates get broken down into (primarily) glucose, which gets imported into cells and undergoes metabolism in order to generate ATP.
--this process begins with glycolysis (which can occur on its own during anaerobic conditions) and leads to the TCA cycle + oxidative phosphorylation/electron transport chain (the latter two processes occur during aerobic conditions)
--excess sugars are "stored" in the body in the form of glycogen (glycogenesis)
When we are fasted, we need to burn glycogen in order to release glucose, so that we can use that glucose in order to make ATP using the processes listed above (this is called glycogenolysis).
Alternatively, we can synthesize glucose from scratch in a process called gluconeogenesis.
