Judicial review States that the Judicial branch has the power to decide whether something is or isn't constitutional. This means for any action of any citizen of the USA. This includes political officials and government employees.
One example was the Supreme court case Marbury v Madison. This case involved an act of congress to be unconstitutional. Learn more
Another example of Judicial review was Dred Scott v. Stanford. This described slavery in the territories and how the US government was to deal with it. Learn more
Finally there was the case of Fletcher v Peck. This case decided that a state law was unconstitutional. Learn more