What exactly is boring about Sartre? Sartre is fantastic. This is an extremely oversimplified, abridged version of Sartre’s basic philosophical introduction of existentialism.
Sartre develops the notion that essence precedes existence. Consider the example of a chair. It is not as though a chair exists, and then we contemplate what the chair is, or what is function might be. Instead, we conceptualize the idea of the chair, and then we develop and create it. The essence, or the concept, must be constructed before the object itself can emerge or exist.
However, if we accept the nonexistence of god (Sartre said to Forlorn that we are condemned to be free), and we consider the emergence of humanity, then the original notion appears in the contrary. Existence precedes essence. Humanity did not choose to emerge, nor is there a description of our purpose or our design. Instead, we were forced into existence, and we have defined ourselves. We are at first nothing, but once we have realized that we are nothing, then that is when we decide what we will be. He said that “subjectivity is the starting point,” and that we must first realize that we have the ability to define ourselves. Each individual must come to this realization, before they can construct an independent definition of themself.
There is no objective purpose of humanity, nor is there a specific function. It is our own subjective perception and model of reality, that dictates what our independent purpose and function is. Our existence is nothing more than the models of experience that we have constructed, and our subjective perception of those models. Man is not connected to a priori definitions, or connotations of what it is that we are.
However, this creates an issue of morality. If we are free to choose our purpose and function, what stops us from killing each other, or destroying everything that we decide necessary? Since humanity is defining itself independently, and we are developing our own subjective notions of who we are, we are at the same time defining humanity as a whole. The objective perspective of humanity, exists as a co-operative group of each of our individual sets of subjective perceptions. Morality is derived from our ability to choose our interactions with reality; and our ability to choose is derived from our human freedom. We must not negatively impact or interfere with another individuals ability to choose freely, as that will affect our ability to choose freely. The individual consciousness is responsible for its own actions, and choices that it makes, regardless of the consequences. We are condemned to be free because our actions and choices are ours alone, and we are condemned to be responsible for our free choices.
It's been a while since I have read Satre, so feel free to correct my understanding.