The title The Stranger refers to the novel's protagonist, Meursault, an alienated young man who comes to view life and existence in general as absurd. Meursault is detached from everything and everyone around him. He experiences no grief when his mother dies, even though the two were not on bad terms. He does not love his girlfriend, Marie. He has no real friends. He subscribes to no religious creed or moral belief.
For the author, Albert Camus, all of this means that Meursault does not "play the game," as it were. That is, he does not pretend that life has meaning, and he does not graft an artificial order onto his life in order to force meaning upon it, whether that system of order is Christianity or the law of the land. He is only interested in his own physical comforts, such as getting out of the hot sun or enjoying sexual pleasures with Marie. This position is greeted with baffled hostility by others in the book. As a result of this, Meursault is branded a monster because he makes no attempt to hide his profound disconnection from a society obsessed with meaning and morality. Therefore, the title is a reference to Meursault's being estranged from society at large, especially from its impulse to contrive meaning and order.