When Sartre writes that “man is condemned to be free,” he is elaborating on the statement “[man] is responsible for everything he does.” Provided that God does not exist, man must live his condemned life of freedom without “any values or commands that could legitimize [his] behavior.”
Furthermore, man has no means of “justification or cause;” man has no excuse for his actions, because he has chosen them on his own, out of his own freedom. Although freedom is traditionally characteristic of “good,” Sartre describes it almost as a burden, because of his belief that God does not exist. When man discovers that he is “without excuse,” he has come to realize that he “cannot find anything to depend on;” for without God, everything is “permitted.”
Also, because there is no outside force driving every man, each man as an individual is responsible for shaping the image of man, or “invent[ing] man.” Assuming that God in the heavens is not laying out the future and that we are all responsible for ourselves, at “every instant” man is condemned to “invent man,” for there is no definition of man except what we make it [pg. 380, textbook].
Furthermore, Sartre explains how each man creates his own morality. Because every man is free to do what they want and make their own decisions, they sometimes are put in a situation where they must choose between “two different modes of action” [pg. 381, textbook]. If God existed and made the future as he wanted it, man would not have to make these decisions. In choosing between these two paths, man must create what he really believes in. This is choosing morality.
In choosing, man develops his own morals and beliefs which result in his individual path of principles. Obviously, in some situations you may have chosen differently than another person, proving that something that means a lot to one person may in turn mean nothing to you.