That’s easy:
“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...”
What part of “natural born” do you not understand?
I would like to add a quote by Thomas Jefferson “We can say with confidence that a natural born citizen of the U.S. means those persons born whose father the U.S. already has an established jurisdiction over, i.e. born to fathers who are themselves citizens of the U.S.” 1777