According to this ruling, the government can regulate speech if the words that were spoken (or printed) represent a "clear and present danger" to American society.
This ruling came as a consequence of the case Schenck v. United States (1919), in which the general secretary of the U.S. Socialist Party opposed the implementation of the military draft. He distributed pamphlets encouraging people to oppose the draft. The unanimous decision was that his words represented a clear and present danger to the country, and therefore his arrest was justified.