What is a judicial review? how is it an example of checks and balances?

Judicial review- seperation of power in modern governmental system.

In the United States, judicial review is considered a key check on the powers of the other two branches of government by the judiciary.

The Supreme Court is able to assess the constitutionality of a law through a process known as "judicial review." 

Since the Judicial Branch is one of three main branches within the United State's Government, it serves to counter-act the Executive & Legislative by putting them under the scrutiny of another branch, and vice versa.