It's not. In the US case it only applies to persons from American Somao. (Of which there is a large community here in Killeen Texas, because so many of them choose to join the US Army).
But for other nations, there may be a significant number of people who are nationals but not citizens. Citizenship means that one is a full participant in the national life, nationals are represented abroad by US diplomats, but do not have the full rights of citizenship.
American Samoa. (Plus some little island in the Pacific were no one currently lives full time.)
If you go back to the time of the founding of this country, this seems to be what they referred to as denizens instead of citizens, and Obama would neatly fit in that category as the son of a foreign national ... if indeed he was born in the United States. In contemporary terms, he might be a national but not a citizen and certainly not a natural born citizen.