The decision of the Supreme Court was that the executive privilege of an American President has limits.
After the Watergate Scandal event happened (June 1972), in which seven of Nixon's closest aides were caught wiretapping phones and stealing documents, President Nixon refused to show the evidence before a Court, claiming his "executive privilege", that is to say, his right as President to withhold information from other government branches to preserve confidential communications within the executive branch or to secure the national interest.
However, the case decided that the executive privilege wasn't absolute, and had limits. It wasn't immune from judicial review, regarding the demands of due process of law. The case ordered the President to deliver tape recordings and documents to the Supreme Court.