In theory and in practice, courts can use the power of judicial review to rule laws unconstitutional.
Judicial review is the power to examine the actions of the legislative, executive, and administrative arms of the government and to determine whether such actions are consistent with the constitution.
Actions judged inconsistent are declared unconstitutional and, therefore, null and void. The institution of judicial review in this sense depends upon the existence of a written constitution.
For example if Congress were to pass a law banning newspapers from printing information about certain political matters, courts would have the authority to rule that this law violates the First Amendment, and is therefore unconstitutional. State courts also have the power to strike down their own state’s laws based on the state or federal constitutions. - NationalParalegal.edu - Judicial Review.
To view a summary of the concept of checking and balancing power, see Introduction to Checks and Balances in Federal Government from RegentsPrep.org.