The president can veto a bill. However, the congress can override that veto only by two-thirds vote from both the senate and the House of Representatives. If that ever happens, it becomes a law over the President’s objections.
The United States Consitution on its Article I, section 7 enacted in the president the power to veto a bill passed by Congress. This is part of the regulation between the three branches of the Federal government.
The veto is one of the most significant powers the Executive branch posses. It gives the president the authority to cancel a law passed by Congress. He/She has ten days since the law is passed by the Congress to present in written form a "veto message" stating why the law must not be enacted.
Congress could agree with the president's decision or override its decision by voting the law again with two-third votes from each house, that way the bill became law without the president´s agreement.
