I'm not sick,but I'm not well.

Existentialism is a branch of philosophy that encompasses a wide variety of views, so it could mean... a lot of different things for someone to claim that label! For more, I've talked about it here.

The central tenet is that understanding life and meaning and purple starts with existence. We exist, so what? Existentialism then emphasizes choice--your choices make you who you are; your choices give meaning to your life. You aren't born with innate destiny, but instead create your own destiny.

However, what that means can be very, very different. Some interpret it as always meaning existential nihilism, wherein there is no meaning but you can create meaning yourself, be it through the concept of life itself or existence. You also have existential humanism, existential atheism, and on and on it goes. I'm more of a Christian existentialist humanist a la Dostoyevsky or Kierkegaard--I think we're all endowed with the image of God at birth, but I also think that a key part of that is our ability to choose and create meaning.