The work is an introductory lecture to the major themes of Sartrean existentialism. Thus, many of Sartre's most important concepts appear throughout the book in a more simplified form than in his other works.
One example is the notion of "existence precedes essence," developed in his previous books. It states that humans do not have an innate "essence," which distinguishes them from other beings. While the life of an animal is determined by its nature, and the meaning of an object by its function, according to Sartre, human life is not determined by anything. Thus, humans exist in the world first and create the meaning for their own lives through their choices and actions. As Sartre states, "man first of all exists, encounters himself, surges up in the world – and defines himself afterwards."
Furthermore, Sartre presents his critique of determinism, which he calls "deterministic excuses." In that sense, Sartre also presents a fundamental concept for his thought in Existentialism Is a Humanism, the notion of "bad faith," which would be an attempt to deny one's own responsibility for their choices. Thus, those "deterministic excuses" would be a way of practicing "bad faith."
This absolute responsibility the individual has for their actions would then cause "anguish," a feeling of loneliness and despair, very cited in Sartre's works. Considering that, Sartre criticizes Christians by stating that their reliance on God is "self-deception" and a way to flee anguish. For Sartre, above all, existentialism is a philosophy of freedom and action. So, the Sartrean existentialist is the individual who takes responsibility for their own choices and creates his own essence throughout life.