"Citizens United" refers, as Joan says, to a court decision. The content of the decision was to extend the legal fiction of corporations as people to allowing corporations the rights of free speech which required allowing the corporation to spend its money to promote any goal it wanted to pursue. This overturned some election finance laws that had previously restricted the ability of corporations to create advertisements for or against candidates.
The sign means "reverse the court decision in the Citizens United vs. Federal Election Commission case of 2010" http://caselaw.findlaw.com/us-supreme-court/08-205.html - the sign does not actually directly refer to the organization Citizens United that Joan linked to. The "corporations are not people" refers to the part of the decision where it states:
The Court has thus rejected the argument that political speech of corporations or other associations should be treated differently under the First Amendment simply because such associations are not "natural persons."