This book advances the notion and necessity of a "committed" literature. Reading is not a light hobby, nor is writing. The function of a committed writer is to reveal the world so that every reader loses her innocence and assumes all her responsibilities in front of it.
The world, in the sartrian existentialism, is in fact the action's field of individuals. The writer shows the world as human project (which reminds the reader of what Heidegger said about the human existence as a project, Entwurf, which is pro-jected or geworfen).
And it is exactly because of the projectual nature of the world that individuals can be involved and engaged in a project of change.
Writing is then an "appeal" to the freedom of the readers so that they committ themself to make the world more and more impregnated with the freedom.
Every literary work suggests a concrete liberation starting from a particular alienation, because the man is always "in situation".
The theory expounded in "What is Literature?" excludes the poetry from committed arts. Only the prose can be committed because there the words are sign of an aspect of the reality, whereas the poetic words are auto-referenzial and so opaque toward the world. If you think of a surrealist, dadaist or futurist poetry, one can clearly understand this exclusion--though it's probably more difficult to agree with it in the case of Dante's poem.
Readers familiar with Heidegger's late writings about poetry, will not appreciate this part of the book.
Knowledge or at least familiarity with Sartre's "Being and Nothingness", his masterpiece, will help the reader to gain a better understanding of this interesting book.