Freedom of speech
Freedom of speech in American law refers to the recognition by the Founders that government interference in the reporting of facts and expression of opinions in lectures, speeches, sermons, street harangues and private conversation frequently does more harm than good. They therefore prevented Congress from enacting any law that would curtail this freedom.
As originally intended, the freedom of speech never meant a perversion of conventions of polite society. You can't insult someone or disturb a religious service with impunity, for example. And of course deliberately creating a panic by falsely shouting fire in a crowded theatre is no more a freedom of speech issue than incitement to riot. The principle refers to what you can say, not how or when you can say it.
The American concept of verbal freedom contrasted sharply with that of England and other monarchies of the time, where to make a disloyal public statement was punished as treason. Communist countries in modern times, likewise, define as treason or sedition any criticism of government policy. Not so in America.
In 1734, John Peter Zenger, the owner of the New York Weekly Journal, published a series of articles critical of the royal governor of New York. Zenger was charged with criminal libel. At trial, the defense showed that the criticisms of the governor were true, and argued that no one should be punished for truthful criticism of public officials. The jury found Zenger “not guilty.” Ever since, the case has stood as an early American assertion that freedom of speech is, and ought to be, a principle of law. 
In some circumstances, freedom of speech can be criticized for its allowing to spread dangerous lies that ultimately harm our values. "McCarthyism" was justified by the imminent communist threat on America.