The best explanation for the phrase "existence precedes essence" is that an individual has no human nature prior to his or her existence.
Jean-Paul Sartre explained it himself in Existentialism and Human Emotions, when he said: "What is meant...by saying that existence precedes essence? It means that, first of all, man exists, turns up, appears on the scene, and only afterwards defines himself... [A]t first he is nothing. Only afterward will he be something, and he himself will have made what he will be."