What is the First Amendment and why did 3 news outlets sue Trump?

After losing access to the White House, three national news outlets filed a lawsuit against President Donald Trump and members of his administration. The legal action comes after Trump announced that he would ban CNN, MS NOW and Politico from access. The announcement came Friday. The next day, reporters from the three outlets were turned away when they tried to enter the White House, and their credentials were deactivated. This Monday, the organizations went to federal court. The dispute p...