The context for this quote is a 1946 lecture by Sartre, published as an essay entitled "Existentialism is a Humanism." In this essay, Sartre responds to criticisms of existentialism from a number of different quarters. These criticisms, he writes, are rooted in the idea that existentialism is a philosophy that...
is excessively individualistic. Detractors say that Sarte denys that people have obligations to each other.
In the paragraph from which the quote is taken, Sartre argues that these views, shared by critics as diverse as communists and Catholic theologians, misconstrue what existentialism is supposed to mean. Sartre, as a self-described atheistic existentialist, asserts that man has no essence. In other words, we are what we make ourselves. For him, this means that mankind has a choice to become what it wishes to be. Man is not inherently good, nor evil, nor sinful—these words have no real meaning to Sartre. When a person makes a decision to be something, that is, when they do anything, they are, according to Sartre, making a decision about what "he believes he ought to be...To choose between this or that is at the same time to affirm the value of that which is chosen."
Sartre meant to turn an old philosophical idea on its head by positing that a person's existence comes first, and their essence comes second. Christians argued that people, created by God, had an essence that came from this creation. They could be understood primarily as beings created in the divine image, but doomed to damnation because of original sin if not given grace by God. Marxists believed that people could be understood objectively by their economic position in society.
Sartre argued that a person was only that, a person. They were defined by what they did, not what they were a priori. For Sartre, this was empowering. If a person wanted to change the world, they were free to do so through literally every single one of their actions. In this way, these quotes show how Sartre defined his worldview not as nihilistic, but in fact as optimistic. As he writes later in the essay, "the destiny of man is placed within himself," as he believed that everyone could affect each other through their choices. Humankind could not be understood in terms of their relationship with the divine, nor to the means of production, nor even to each other. Rather, each person is an individual, and exists unbounded in their moral and ethical choices.