Article Two of the United States Constitution
Section One begins by giving federal executive power to the President only.  This is part of the separation of powers that the Founding Fathers built into the Constitution.   To prevent any part of the government from getting too powerful, they split the power between three branches.  This clause gives executive power to the President.  Another clause in Article One of the Constitution gives federal legislative (law-making) power to the United States Congress only.  A third clause in Article Three gives judiciary power to the federal courts.  No branch is allowed to do a job that the Constitution has given to another branch.  For example, the President cannot make laws; that is the Legislature's job.