Judicial review is the power granted to the judicial branch of the American government to review and invalidate the actions of the legislative and executive branches.
Judicial review may only be practiced as part of rulings in cases that come before the courts.
Judges have no power to officially invalidate the actions of another branch of government if those actions do not form the basis of a genuine case or controversy.
Judicial review is a fundamental power given to courts to ensure that the proper checks and balances operate in the American system of government. If courts could not invalidate laws or executive actions that were unconstitutional, then the Constitution would be little more than a set of suggestions.
At the same time, the case or controversy clause of Article III of the Constitution restricts the power of judges to only real-world situations, and prevents them from proactively invalidating laws.