Sartre's Existentialism

Jean-Paul Sartre was one of my favorites when I was an undergraduate. I enjoyed his novels and plays, and his great essay “Existentialism as Humanism. “ And I once even read a good bit of "Being and Nothingness," his 700 page magnum opus. So what did Sartre mean by saying that we are radically free, and that we are condemned to be free? And what is existentialism?