The main purpose of the system of checks and balances in the United States Constitution is to ensure that no one branch of the American government becomes more powerful than the others. This is also referred to as the separation of powers. While this model's roots can be traced back to ancient Greece, the American Constitution borrows heavily on this idea from the French Enlightenment philosopher, Baron de Montesquieu.
The three branches of government include the executive, judiciary and legislative. The executive branch includes the president, the judiciary includes the Supreme Court and the legislative includes Congress. Power is limited in many ways, but an example of checks and balances with regard to the legislative branch includes the president's ability to veto a law that is passed by Congress. Similarly, the judicial branch, or the Supreme Court, may deem a law that is passed by Congress to be unconstitutional. While the president appoints members to the Supreme Court, the appointments have to be approved by Congress.
There are many roles of each branch of government, but every function of each role is designed so that it is checked over by another branch of government. For example, while the judiciary branch can proclaim interpretations of laws by Congress, they cannot change them.