"Every Bill which shall have passed the House of Representatives and the Senate, shall, before it become a Law, be presented to the President of the United States: If he approve he shall sign it, but if not he shall return it, with his Objections to that House in which it shall have originated..."—U.S. Constitution
This quote directly reflects the principle of
federalism
democracy
popular sovereignty
checks and balances
The president checked what Congress did, he didn't like it, or it's unconstitutional, he veto's the bill which creates a wealthy balance, between the 3 branches of Government.