The title is significant on a number of levels. Meursault, the book's protagonist, is the eponymous stranger—a stranger to himself, to his family, to his country, and to anyone with whom he comes into contact.
Cut off from all meaningful interactions with other people—indeed, cut off from any kind of meaning at all—Meursault is the ultimate existentialist antihero and a man who attempts without success to impose some semblance of meaning on his existence by taking up a firm, resolute attitude to life. On Meursault's interpretation of existentialism, resolute action is all-important, irrespective of the content, even if it entails the senseless killing of another human being, as in his case.
But far from being an authentic act, Meursault's killing of the Arab doesn't make him less of a stranger to himself. This is because, in time-honored existentialist fashion, he has no core to his being: no essence, as it were. All he has is existence—that self-created existence which he constantly needs to recreate, irrespective of the circumstances.
The constant need to do this drives a wedge between Meursault and the alter ego he's created, meaning, among other things, that he will never truly be at home with himself and will always be a stranger, both to himself and to others.