and in case, like me, your Sartre reading was too long ago, here's one xplanation:
A central proposition of Existentialism is that existence precedes essence, which means that the most important consideration for individuals is the fact that they are an individual—an independently acting and responsible, conscious being ("existence")—rather than what labels, roles, stereotypes, definitions, or other preconceived categories the individual fits ("essence"). The actual life of the individual is what constitutes what could be called their "true essence" instead of there being an arbitrarily attributed essence others use to define them. Thus, human beings, through their own consciousness, create their own values and determine a meaning to their life.