In the link reaction if oxygen is present , the pyruvate that is formed by glycolysis enters into the matrix of the mitochondria where it forms the link reaction. It is termed the link reaction because it links glycolysis to the krebs cycle. The 3carbon pyruvate loses a carbon, in a decarboxylated reaction where the single carbon forms CO2 , while the 2C compound (called an acetyl group) attaches to a coenzyme A. The CoA (coenzyme A) with the attached acetyl group is called acetly CoA. The pyruvate is also being oxidised at the same time, losing hydrogen to NAD forming reduced NAD.
Now the reduced NAD can be oxidised in the electron transport chain to produce more ATP. But the acetyl group that still contains energy from the original glucose molecule, will be released in the Krebs cycle.