NADH and FADH2 are electron carriers. They pick up electrons at one place and drop them off at another.
For example, NAD+ is the form without an electron; when it picks up an electron it becomes NADH. It cycles back and forth between those two forms as it pick up an electron at one place (becoming NADH) and drops it off at another (becoming NAD+).
Sticking with the TCA cycle, the electron carriers pick up electrons there and then carry them to the electron transport chain, where the carriers drop off the electrons. The electron carriers then return to the TCA to pick up more electrons, to carry them to the electron transport chain.
NADH is also formed in the cytosol by glycolysis, so glycolysis requires NAD+ in order to occur. And again, the NAD+ picks up an electron to become NADH, and then carries the electron to the electron transprot chain where it is dropped off. The resulting NAD+ then returns to the cytosol to pick up another electron from glycolysis.
Per starting glucose molecule:
glycolysis -- which is the only step that occurs in the cytoplasm: the rest occur in the mitochondria -- converts 2 NAD+ into 2 NADH.
Conversion of pyruvate to acetylCoA converts 2 NAD+ into 2 NADH.
The TCA converts 6 NAD+ into 6 NADH, and into 2 FAD into 2 FADH2.
All of these NADH and FADH2 molecules drop off electrons at the electron transport chain, then return to pick up more electrons, in a cycle.
PS: The electrons dropped off at the electron transport chain are high energy electrons. As they "fall down" the "chain" they release energy, which is used to pump protons across the mitochondrial inner membrane uphill - against their concentration gradient. This forms an electrochemical gradient across the inner mitochondrial membrane which is then used to produce ATP.
BS in biology; university biology tutor