to protect the rights of the citizens.
While it would be valid to say that the constitution is to protect the rights of the citizens.
The first 10 amendments (The Bill of Rights) was designed to protect the rights of the citizens.
Additional amendments can be defined as either further clarifying citizen's rights, or updating major constitutional issues or further defining the federal government organization.
It essentially provides a template for all US law, and by definition is the set of laws that can not be declared unconstitutional.
While Congress can pass laws to support or clarify constitutional law, they are also the set of laws that Congress lacks the authority to directly modify, and thus provides a check on the powers of the Federal Governmental Bodies.