The system of checks and balances is a part of our Constitution. It guarantees that no part of the government becomes too powerful. For example, the legislative branch is in charge of making laws. The executive branch can veto the law, thus making it harder for the legislative branch to pass the law. The judicial branch may also say that the law is unconstitutional and thus make sure it is not a law.
The legislative branch can also remove a president or judge that is not doing his/her job properly. The executive branch appoints judges and the legislative branch approves the choice of the executive branch. Again, the branches check and balance each other so that no one branch has too much power.