The system of checks and balances is designed to keep any one branch of the U.S. government from becoming too powerful. It gives each branch specific ways to counter actions of the others, ensuring that no single section ever has complete control.
The three branches of the U.S. government are the executive branch (the president), the legislative branch (Congress) and the judicial branch (the Supreme Court). Each branch has its own specific role to play and unique set of powers. Within these powers lie some form of oversight over the other branches, known as the system of checks and balances.
For example, bills and laws are proposed, drafted and voted into effect by Congress. However, the President has the power to veto a bill, blocking it from going into effect. Congress also has the ability to override that veto if two thirds of its members vote in favor of the override. The Supreme Court also has the power to declare a law unconstitutional. This is kept in check by the fact that Supreme Court members are chosen by the president and must be voted into office by Congress. This system ensures that at all points, some section of the government is providing oversight and regulation of the others.