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Did You Know?
Jean-Paul Sartre was honored with the 'Nobel Prize in Literature', in 1964. However, he humbly declined the prize, and in the official statement given to the press he justified his refusal, since he personally opined that accepting awards would institutionalize writers.
As human beings, we are constantly looking out for the purpose of our existence. After a human is born out of the protective shell of a mother's womb, we enter the world completely clueless and oblivious of what is in store for us. As we grow, we're influenced by others: parents, teachers, society, etc.

However, when the question of purpose of our existence comes into the picture, we all are often clueless about its answer. Philosophers who have studied human life, have their own set of explanations and theories. However, 'existence precedes essence' is one of the most famous phrases quoted by Jean-Paul Sartre, a French philosopher and writer. An advocate of existentialism, Jean-Paul Sartre's quote goes against the traditional philosophical viewpoints of values and purpose of life.
Meaning of the Phrase
"What is meant here by saying that existence precedes essence? It means first of all, man exists, turns up, appears on the scene, and, only afterwards, defines himself. If man, as the existentialist conceives him, is indefinable, it is because at first he is nothing. Only afterward will he be something, and he himself will have made what he will be."

― Jean-Paul Sartre, Existentialism Is a Humanism
Consider a simple example. When we think of any object, its purpose is determined beforehand. It will not exist or be made unless it has a purpose. An artist will first frame a picture in his mind, and then, accordingly make that painting. A chair has its own purpose determined first, and is hence manufactured, i.e., brought into existence in this world. On the other hand, human beings naturally do not have any nature or essence to themselves. They simply exist in this world, and then, consciously, their choices make them the person they are.
Existence: Man is thrown in this world, of which he has no knowledge. He is born 'nothing'.

Essence: His personality is a product of his own choices. He has to bear responsibility of his choices, and constantly make a choice. He determines his own essence.
► Before developing the essence, humans are thrown into existence first. We do not have the choice of the world where we exist. However, our essence is determined by our own choices; we are a manifestation of our choices.

► Perhaps, if we blindly follow others, only trying to 'fit-in', it is because we do not have the courage to live by our own thoughts and actions. We always have a choice, and we do not exercise it due to fear in our minds. We simply let others influence us.

► We are responsible for our actions. Though we are born free, and freedom is a basic human right, we must choose and bear responsibility for the same. We cannot transfer the liability of our actions onto someone else.

► Man exists first, and then chooses his nature through his choices.

► Individualism and freedom are the pillars of existentialism.

► Why is there despair? Humans cannot predict the consequences of their choices, thus, they can never be emancipated from this decision-making process and also claiming responsibility to it.
Trivia
Jean-Paul Sartre lived with his partner Simone de Beauvoir, who was also a French philosopher, and a feminist. Their relationship was quite progressive, as compared to those times.
'Nausea' (Novel) by 'Jean-Paul Sartre'
'Nausea' is a story of Antoine Roquentin, a historian, who is researching on a French aristocrat named 'Marquis de Rollebon'. However, he is suffering from a sick feeling which he calls 'nausea'. Later he discovers that this feeling has taken birth from his doubts of his existence, and he is probably influencing himself by Rollebon for his existence. After a futile attempt at trying to have a conversation with his love interest Anny, he distances himself more and more from this world, feeling lost and helpless about his existence in this world. The book further explains Sartre's further view points about man's existence.
In short, the phrase implies that, what we become is a result of our own choice and accord. We cannot put the blame on others, since we have the freedom to exert our own choice and determine our nature. We are simply born as nothing, colorless, and without any personality characteristics. With our growth, we determine the purpose of our own existence. And with this freedom comes the responsibility to do something.