Well, basically, they accept the high energy electrons that come from fuel sources that we ingest into our bodies, including sugars and fatty acids. So NAD plus and Fad plus molecules accept electrons and they carry these high energy electrons until the electron transport chain found in the inner mitochondrial membrane. And the electron transport chain uses these high energy electrons to synthesize ATP molecules. So we see that although NAD plus and Fad plus molecules are not a direct source of energy, they are used to actually form the energy molecules that are used by the cell, namely adenosine triphosphate or ATP molecules. Now, when these electron carrier molecules accept our electrons, they are reduced. So the fully reduced form of NAD plus is NADH, and the fully reduced form of Sad plus is Fadh too.