Gluconeogenesis (pronounced as glue-ko-neo-gen-ah-sis) is the conversion of non-carbohydrate sources (pyrovate, amino acids from protein, and glycerol from fat) to glucose. Your body uses glucose, also called blood sugar, as fuel, and it's especially important for your brain, red blood cells and muscles to function normally.
Under normal circumstances, your body gets glucose by breaking down carbohydrates that come from the foods you eat. It uses whatever amount it needs as fuel, stores some in the muscles and liver as glycogen, then converts any leftover glucose to fat. If you don't eat enough carbohydrates, your blood sugar level will fall. When it gets to a certain level, a hormone called glucagon will trigger gluconeogenesis in order to bring that level back up.
Any time you reduce your carbohydrate intake or significantly increase your physical activity long enough, gluconeogenesis will occur. This can happen during a low-carb diet, fasting, or during starvation.