Bad faith in existentialism is when you don't act like yourself and instead pretend to be someone you are not. It's like wearing a mask or playing a role instead of being true to who you really are.
Imagine you really love Batman, but your friends keep teasing you for it. You might pretend to not like Batman and instead try to fit in by pretending to like something else. This is bad faith because you are not being true to what you enjoy or who you are, but instead you are pretending to be someone you are not just to fit in.
In life, bad faith can happen when we try to live up to other people's expectations or societal standards, and we stop being true to ourselves. We might start to believe that we need to act and think in a certain way just to be accepted, even if it doesn't feel right. This can lead to feeling unhappy or unfulfilled because we are not living according to our own values and desires.
Existentialist philosophers argue that the only way to live authentically is to be honest with ourselves and make choices based on our own beliefs and values, even if it's not what others expect of us. So always be yourself, even if others might not like it!