If you limit yourself to Lewis structures and resonance structures this is true. With 10 total pi electrons in the naphthalene system, if one ring "has" 6 and is benzenoid, the other can only have the 4 that are leftover and is more of a "diene". That is why your organic chem prof will dock you if you draw naphthalene with a 6 pi electron circle in each ring - that means it has 12 pi electrons when it only has 10. If you are in sophomore organic chemistry - you can stop reading here.
There is some truth to this - it is easier to hydrogenate the first ring of naphthalene than the second, and far easier than to hydrogenate benzene. But it is also misleading - the electrons are not required to localize as the Lewis and resonance structures require - they can go wherever they want.
If you look up a treatment with quantum mechanics / molecular orbital theory you will see how the orbitals stack up by energy. The lowest orbital has in phase contributions from all carbons around the perimeter, the other 9 orbitals have various combinations. The lowest 5 are filled, the highest 5 are empty.