Krebs cycle

The Krebs cycle (named after Hans Krebs) is a part of cellular respiration. Its other names are the citric acid cycle, and the tricarboxylic acid cycle (TCA cycle). 
The "Krebs cycle" is a series of chemical reactions used by all aerobic organisms in their energy conversion processes. It is important to many biochemical pathways. This suggests that it was one of the earliest parts of cellular metabolism to evolve.
The Krebs cycle comes after the link reaction and provides the hydrogen and electrons needed for the electron transport chain. It takes place inside mitochondria.
Summary.
The diagram below shows how this part of respiration is an ever-repeating cycle which produces ATP and gives off CO2. The ATP is a molecule which carries energy in chemical form to be used in other cell processes. To summarize: 
Because two acetyl-CoA molecules are produced from each glucose molecule, "two cycles are required per glucose molecule". Therefore, at the end of two cycles, the products are: two ATP, six NADH, two FADH2 two QH2 (ubiquinol) and four CO2.