Constitutional supremacy means that no laws or actions can violate a nation's constitution. It can exist in countries with a broad range of governments. However, constitutional supremacy only applies if the government and military enforce it.
Constitutional supremacy is viewed as a check on governmental power. No matter who is elected, the constitution's principles must be enforced. In the abstract, this prevents a wide range of potential government abuses. In practice, governments may ignore aspects of their nation's constitution or interpret them in different ways.
Constitutions can be amended, but the requirements for amending constitutions tend make doing so difficult. A constitution that can easily be changed lacks the protections of one that requires a broad consensus to change.
While constitutional supremacy is designed to be strong, upholding it can be challenging. In many cases, people have no means of enforcing it. If a court states that the government is in violation of part of the constitution, it likely has no means of enforcing its decision. In some cases, the government might simply ignore these rulings. In many nations, the military operates with some degree of independence. If the military chooses to allow violations to continue, revolt may be the only solution.