It doesn't say that anywhere. We made it up. This is what Marbury v. Madison said. And what the law means is the job of courts. They have to say it all the time.
Courts frequently have to try to reconcile conflicting statutes, for example.
In doing so, they have to interpret them. If they cannot reconcile them, they simply say the more recent one prevails over the older one. It is assuredly the function of the courts to say what the law is. It's lawyer's work. But if that is not what the Constitution is, if it is not a text, like a statute, which means what it meant when it was passed.