The first ten amendments to the Constitution of the United States are collectively termed the Bill of Rights. Introduced in 1789 by James Madison as a way of revising the Constitution to protect individual liberties, the Bill of Rights eventually was ratified as a group of Constitutional amendments on December 15, 1791.
The First Amendment reads:
Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof; or abridging the freedom of speech, or of the press; or the right of the people peaceably to assemble, and to petition the government for a redress of grievances.
Its first purpose was to guarantee that the United States would not have an official state religion, and would allow people to freely choose whether to follow any or no religion as they chose.
The next part of the amendment guarantees freedom of speech and the press, meaning you can say or publish what you wish without fear of being persecuted.
Finally it guarantees that people can assemble however they wish (as long as they don't riot) and send petitions to the government when they disagree with its decisions. In other words, while treason or violent revolution is illegal, it is perfectly legal for citizens to disagree with the government and express those disagreements individually or in groups.