Sartre thought that meaning is essentially unfixed. Humans give an order and meaning to the world. We do not make the world, but we do give it meaning. To use the old example, if a tree falls in the forest and no one is there to hear it, it does make a sound, but it only means something when someone (a consciousness) is there to hear it and put it into a meaningful context.
As this project of making meaning via our relation to the world is never finished, a work of art is never finished. In fact, Sartre proposes that reading is similarly a never-ending, creative practice. Reading means something only as it is done. The tree falling in the forest means something only when it is heard. Reading any text means something only as it is read. "Beyond that, there are only black marks on paper." (from What is Literature: "Why Write?")
The writer writes to be read. Referring to the analogy (given that the tree is conscious), the tree falls to be heard. The inauthentic reader reads as if he is a blank slate upon which what he reads is imprinted like a photograph. The authentic reader doesn't passively receive the author's perspective. Rather, the authentic reader treats reading creatively:
And if I am told that it would be more fitting to call this operation a re-invention or a discovery, I shall answer that, first, such a re-invention would be as new and as original an act as the first invention.
This ties in with Sartre's existentialist maxim that existence precedes essence. That is, we exist and then we create our own essence (who we are). Clearly, the physicality of the world and social structures act as a guide (just as a text's author acts as a guide to that text), but we have the opportunity, the existential responsibility, to create or re-invent meaning ourselves: subjectively. Therefore, Sartre endorses and encourages the freedom of the reader as much as he endorses the freedom of the individual in the world.
Sartre proposes a dialectic relationship between the writer and the reader. The author has creative freedom in writing just as the reader has creative freedom in reading: "the more we experience our freedom, the more we recognize that of the other; the more he demands of us, the more we demand of him." And in a more fundamental sense of existence, as we acknowledge each other, we acknowledge each other in existence. To write is "to have recourse to the consciousness of others in order to make one's self be recognized as essential to the totality of being . . . "
And the author's whole art is bent on obliging me to create what he discloses, therefore to compromise myself. So both of us bear the responsibility for the universe.
We can not fully know what a writer intends. We certainly can not know why God made the world as such. So, we are left with the opportunity and responsibility of making meaning. Sartre agrees with Heidegger in that the authentic person asserts his/her freedom in challenging the way we are guided to think "X" and rather than passively receive the marks on the page, the authentic reader re-invents them as he reinvents himself in relation to the world.
This freedom of the reader Sartre discusses is similar to Reader Response criticism which focuses on the reader's subjective response to texts.