Perhaps one of the most famous lines attributed to Sartre is "man is condemned to be free; because once thrown into the world, he is responsible for everything he does." Sartre's notion of freedom comes from the idea that there is no human nature or god to explain our actions....
Everything we choose to do is ultimately at the core of ourselves. Without a god or human nature, there are no moral codes that we necessarily have to follow, or biological directives that guide or provide reason for our actions. Thus, everything we do, we are responsible for. Some may attempt to remove this freedom by joining a religion or groups with specific moral directives, but Sartre calls this "bad faith" in that it denies the essential fact of our being: we are free.There are various ways to think about this. On the one hand, it is scary to imagine a world where the meaning of every action we do resides within us. There is no objective reason to live; there is no objective way to measure right or wrong associated with our actions; there is no objective guide that can tell us what makes a good life. In this way, Sartre's theory is potentially nihilisitic and depressing because we never know for sure what we're doing, and there is no inherent reason for doing it.
On the other hand, his view is empowering. We get to decide what to do with our lives, determining for ourselves what is personally meaningful to us. He calls existentialism a humanism, and aligning with Marx, suggests that we can devote our lives to such worthy causes as mitigating suffering. If existence precedes essence as existentialism claims, it does not matter that there is no objective right or wrong. Instead, we can improve our time on this earth, living for ourselves rather than some god or strict moral code.
Ultimately, I don't agree with his claim to freedom one way or the other, as I am a social determinist. I believe that our actions are determined by our social upbringing and, to a lesser degree, biology, and thus we are not actually condemned to be free. Instead, we are condemned to live out various ideological and discursive states that permeate us from birth.