The memo outlined legal justification for the September 2011 killing in Yemen of Anwar al-Awlaki, an al Qaeda leader who was born in the United States. But parts of the 41-page legal opinion were redacted, prompting criticism from transparency advocates who pressed for a full justification.
The memo concluded that al-Awlaki's citizenship would not preclude the U.S. from "taking lethal action" against him, based on facts about the case submitted by the CIA, Department of Defense, and intelligence community. This summation of legality is predicated on the U.S. government's declaration of al-Awlaki as an "operational leader" of an "enemy force" - al Qaeda. David Barron, then the acting chief of the Justice Department's Office of Legal Counsel, wrote that lethal force would be legal under a 2001 authorization of U.S. military force.
You can read the full, unredacted statement from the Congressional Progressive Caucus here.