Article II, Section 1 of the Constitution reads as:
"No Person except a natural born Citizen, or a Citizen of the United States, at the time of the Adoption of this Constitution, shall be eligible to the Office of President..."
The Constitution, however, does not specify/define what "natural born" means. However, there are two agreed upon definitions:
1) Being born on US soil. Regardless of one's parents, if you are born in America, you are a natural born citizen.
2) If a child is born on foreign soil and at least one of the child's parents is a US citizen at the time of birth, then the child is a natural born citizen as well.
The second definition is sometimes challenged, but that's the other prevailing interpretation. There can sometimes be more requirements if only one parent is a US citizen (like were they at birth? have they lived in the US for a period of time)? For the most part, if both parents are US citizens, then the child--even when born on foreign soil--is a natural born citizen.
Another idea of a "natural born citizen" is someone who is a US citizen at birth and does not have to be naturalized (as in go through a citizenship process).
