The decision in Marbury vs. Madison (1803) was significant because it established that the Supreme Court assumed the power of judicial review over the legislative and executive branches.
Judicial review is the power of the courts to decide whether laws and actions of government are allowed under the Constitution
If the laws and actions of the government are against what is written in the Constitution, then, these laws and actions are deemed unconstitutional. Therefore, null and void or unenforceable.