"Existence precedes essence" is the phrase that defines the philosophy of Jean-Paul Sartre. The basic meaning of this phrase is there is no such thing as human nature or an essence of what is means to be human. There is no preordained plan that makes us the type of beings we are, we have no specific purpose and are free to shape ourselves.
This follows from Sartre's atheism; there is no fixed human nature because there is no God to conceive it. And, according to Sartre since there is no God, we are ontologically worthless; there is no such thing as inherent value, worth or dignity because there is no divine creator to design us and give us meaning. Sartre later argues that his philosophy gives man dignity, it does not reduce him to an object in the mind of God.
I think that is contradictory; I certainly believe humans have intrinstic dignity, value and freedom. Traditional Christian, Jewish and Muslim philosophers would argue that God is the reason for our dignity and freedom. Humans are objects designed by God, but we are also subjects of God, a subject of willing and knowing. But Sartre arives at the conclusion that man is free from the opposite starting point. Then of course, his definition of freedom is unconventional.