If freedom is the inescapable structure of consciousness, and if freedom generates anguish, then it is entirely predictable that humans will spend enormous energy trying to escape it. Bad faith (mauvaise foi) is Sartre's name for the strategies by which consciousness lies to itself about its own freedom.
Bad faith is not lying in the ordinary sense. When I lie to you, there is a liar who knows the truth and a victim who is deceived. In bad faith, the liar and the deceived are the same person. I both know my freedom (consciousness cannot not know itself) and succeed in hiding it from myself. This is only possible because of consciousness's dual structure: I am both my facticity (what I have been, my situation, my body) and my transcendence (my freedom to step back from and project beyond any given situation). Bad faith exploits this duality by overdramatizing one side to make the other disappear.
Bad faith of facticity: I deny my transcendence by identifying myself completely with my situation or past. "I can't help it, that's just who I am." "I had no choice, my background made me this way." "I'm just a waiter. Don't expect more." The woman who accepts a social role as given and natural, who treats her femininity or her class as a destiny rather than a situation she must take up or transform, is in bad faith in this direction. She has converted her transcendence into an in-itself, a fixed thing with a fixed nature.
Bad faith of transcendence: I deny my facticity by treating myself as pure freedom floating above all concrete situations. The person who says "I could be anything, I'm not tied to any of this", refusing to acknowledge that their history, their body, their relationships, their past choices really do constitute who they are, is in bad faith in the other direction. Unlimited transcendence is a fantasy that evades the weight of one's actual situation.
The waiter is Sartre's most famous example of facticity-bad faith:
His condition is wholly one that he has assumed himself. [...] A waiter can be a waiter in the mode of being what he is not. [...] He cannot be a waiter as an inkwell is an inkwell.
— Jean-Paul Sartre, Being and Nothingness, Part One, Chapter II (1943, trans. Hazel Barnes)
The waiter's mechanical precision, his studied posture and solicitous manner, his complete absorption in the role, all of this is an attempt to be waiter as the inkwell is an inkwell: fully, from the inside, with no remainder. But there is always a remainder. He could quit tomorrow. He chose to show up this morning. The performance of total role-identification is itself the proof that he knows he is not merely a waiter, you only need to perform what you actually are.
Now Sartre adds a twist that makes bad faith. Suppose you decide to be completely honest about who you are, you acknowledge your cowardice, your jealousy, your failings, and say "this is what I am." But the moment you claim "I am a coward," you are treating your cowardice as a fixed property of an in-itself, rather than a choice you make and can remake at any moment. True sincerity about your freedom would require saying: "I have been cowardly, and I am free to be otherwise." But this recognition is already beyond what "I am a coward" captures. Sartre concludes, grimly, that "the being of human-reality is bad faith", not that authenticity is impossible, but that it is a perpetual task, never a settled state.sincerity is also bad faith. Suppose you decide to be completely honest about who you are, you acknowledge your cowardice, your jealousy, your failings, and say "this is what I am." But the moment you claim "I am a coward," you are treating your cowardice as a fixed property of an in-itself, rather than a choice you make and can remake at any moment. True sincerity about your freedom would require saying: "I have been cowardly, and I am free to be otherwise." But this recognition is already beyond what "I am a coward" captures. Sartre concludes, grimly, that "the being of human-reality is ==bad faith==", not that authenticity is impossible, but that it is a perpetual task, never a settled state.
Authenticity is the alternative, but Sartre gives it only a sketch in Being and Nothingness (he promised a full ethics and never fully delivered it). To live authentically is to acknowledge your freedom and facticity simultaneously without collapsing either into the other, to choose fully, know you are choosing, and take complete responsibility for what you become without appeal to nature, God, or social determinism as excuses.