Amendment I

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.

What it means?

Freedom of speech, press, religion, peaceable assembly, and to petition the government.

This means that the government can't make any religion an official religion, or stop people from practicing any religion.Also this mean that the government can't stop people from saying what they think. It applies to many forms of expression like complaining about the government and having rallies and marches.