The Constitution protects your right to gather with others and demand change. It is the legal ground beneath every march, rally, and protest in American history.
The right peaceably to assemble is one of the five freedoms in the First Amendment, sitting alongside speech, press, religion, and petition. It protects the gathering of people, in public, to express shared views and press for change.
It is the constitutional foundation of protest itself. A single person speaking is protected by free speech; a crowd of thousands marching together is protected by the right of assembly. Movements need numbers, and this is the clause that protects the numbers.
The protection is not unlimited, but the limits are narrow. Governments can impose reasonable rules on the time, place, and manner of a gathering, a permit, a route, a curfew, but they cannot ban assembly because they dislike its message. The rules must be neutral as to content.
History runs through this clause. The Selma march, the March on Washington, countless demonstrations for and against countless causes, all rest on the right of people to come together in public and be seen and heard.