The Principle of Separation of Powers states that the government is divided between three branches: Judicial, Executive and Legislative. Each branch has a typical power, Judicial power is to judge, Executive is to administrate and Legislative is to make laws. But the Separation of Powers also gives each of those three branches of government atypical powers that exist as a part of the Checks and Balances System.
The Checks and Balances system exists so no branch can have too much power than the others. With the atypical powers, one branch can stop the other from taking too much power, examples of atypical powers are The power of veto of the Executive Branch over laws passed by the Legislative Branch and the possibility to rule law or act unconstitutional by the Judicial Branch.