We know that the two electron carriers NADH and FADH2 donate electrons to the Electron Transport System (ETS) in the mitochondria. There are enzymes that use the energy of these electrons and yield ATP molecules.
But the amount of energy or ATP molecules yielded by NADH is greater than that of the FADH2. This is because NADH enters the Electron Transport Chain (ETC) at an earlier stage/ a higher level (at the first protein complex) as compared to FADH2 (that enters at the second protein complex).
NADH gets oxidized three times and FADH2 gets oxidized 2 times throughout the Electron Transport Chain which explains why NADH yields 3 ATP molecules and FADH2 yields only 2.
Thus, entering the chain earlier means that NADH is able to donate more electrons than FADH2. More electrons mean releasing more energy or formation of more ATP molecules.