The reaction catalyzed by the enzyme glyceraldehyde-3-phosphate dehydrogenase is a coupled oxidation reduction reaction. The glyceraldehyde-3-phosphate becomes oxidized and the NAD+ becomes reduced. The electrons removed from the glyceraldehyde phosphate are relatively high energy electrons. In addition a phosphate group is added to the glyceraldehyde phosphate.
Since each molecule of glucose produces two molecules of glyceraldehyde phosphate, two molecules of NADH are produced in this reaction per molecule of glucose.