Explain how NAD+ and FAD serve as electron carriers when they are reduced &

Recall that reduced means they've gained electrons, so they are effectively 'charged'. Both these carriers are known are nucleotide coenzymes; their main purpose is to transport of protons (H+) and electrons from one reaction site to another.

The reductions of NAD to NADH and FAD to FADH2 are energy-harvesting reactions that will eventually transfer most of their free energy to ATP molecules. The reduced coenzymes act as mobile energy carriers within the cell, moving free energy from one place to another and from one molecule to another.

Now, in your textbook, there should be specific examples as to where these molecules do work. Let me know if you're still having trouble.