Tricarboxylic acid (TCA) cycle is the second stage of cellular respiration. The diagram of this process is shown here https://cdn.britannica.com/83/22483-050-C59E955A/tricarboxylic-acid-cycle.jpg
During the cycle, acetyl-CoA produced by glycolysis and the following prep reaction, is further oxidized. First, acetyl-CoA joins oxaloacetate. The resulting 6-carbon molecule undergoes a series of reactions, when it is oxidized to produce NADH or FADH2. Simultaneously, carbon dioxide is released.
At one of the stages, one GTP molecule, that is then converted to ATP, is made.
In total, there are 3 NADH molecules, 1 FADH2 and 1 ATP that are produced during TCA cycle. These 3 NADH can be further used by the electron transport chain to make 3 ATP molecules each, giving 9 ATP.
1 FADH2 will result in 2 ATP.
Thus, the total energy yield is 9+2+1=12 ATP per cycle.