Checks and balances is a basic principle underlying the creation of American government. It involves the separation of powers, which assigns various powers and functions of government to separate and relatively independent levels and branches of the federal system in order to prevent their all being controlled at the same time by any potentially tyrannical political faction. With checks and balances, each of the three branches of government can limit the powers of the others. This way, no one branch becomes too powerful.
For example, the the President can veto a law passed or repealed by the two houses of Congress may agree on a compromise to pass or repeal a law, the President and Congress can agree on passing a law, but if the federal judiciary declares it to be unconstitutional the courts will refuse to treat the law as valid or enforceable. Although Congress cannot tell a judge how to rule in a particular case before him, Congress has the power to define and redefine the jurisdiction of the various federal courts. The President has general authority over of the conduct of foreign policy and military policy, but his treaties must be ratified by the Senate, and only Congress can appropriate public money to pay for such things as the raising of an army or the dispensing of foreign aid.