We mean that man first of all exists, encounters himself, surges up in the world - and defines himself afterwards. If man as the existential sees him is not definable, it is because to begin with he is nothing. He will not be anything until later, and then he will be what he makes of himself. Thus, there is no human nature, because there is no God to have a conception of it.
Man simply is.
Not that he is simply what he conceives himself to be, but he is what he wills, and as he conceives himself after already existing - as he wills to be after that leap towards existence. Man is nothing else but that which he makes of himself.