The citric acid cycle, also known as the Kreb's cycle, is a part of the metabolism pathway that converts glucose from food into ATP for energy use by an organism. As the name implies, it is a cyclical cycle that accepts a pyruvate molecule from glycolysis and converts it to citric acid. The citric acid then undergoes a series of reactions which ultimately comes back to accept another pyruvate and start the cycle all over again. A single pyruvate molecule produces two molecules of carbon dioxide in a single cycle. Since glycolysis converts glucose into two molecules of pyruvate, one glucose feeds two turns of the citric acid cycle to produce four molecules of carbon dioxide.
It should be noted that one turn of the citric acid cycle also produces one molecule of ATP and three NADH molecules. These NADH molecules are then used in oxidative phosphorylation to produce many more molecules of ATP. So while the citric acid cycle may not produce a lot of ATP directly, it does help feed the next part of the metabolic pathway to produce ATP.