It's the First Amendment. The freedoms of speech, of the press, of assembly, and to petition—discussed here together as “freedom of expression”—broadly protect expression from governmental restrictions.
Thus, for instance, the government may not outlaw antiwar speech, speech praising violence, racist speech, pro-communist speech, and the like. Nor may the government impose special taxes on speech on certain topics or limit demonstrations that express certain views.
It allows an individual to express themselves through publication and dissemination. It is part of the constitutional protection of freedom of expression.
The government also may not authorize civil lawsuits based on people’s speech, unless the speech falls within a traditionally recognized First Amendment exception.
Need a primer as you begin to study the US Constitution? Here's a collection of articles about the Constitution shared by Brighthubeducation.com.