What is this thing Hayek called "the rule of law"?

``... the idea of an unwritten code is pretty much the opposite of what Hayek had in mind when it came to the rule of law. Perhaps Ms Zernike missed the chapter titled "Planning and the Rule of Law" as she read "The Road to Serfdom" in preparation for this article. There, Hayek draws out the difference between "a free country" and "a country under arbitrary government". A country counts as free only if its government is bound by the rule of law, which, according to Hayek, "means that government in all its actions is bound by rules fixed and announced beforehand". ''