What is the significance of the title "The Stranger"?

The title of the book refers to its main character, Mersault. Only a very naive reader could consider that the stranger or the foreigner (another possible translation of the French title) is the Arab, Meursault's victim. Meursault is clearly feeling detached from humanity, which shows at the beginning of the novel, after his mother has died and he cannot remember the exact date and, worse, he does not even care.
Although the word "stranger" does not appear anywhere in the text, it is easy to tell from his own descriptions that Meursault is a stranger among all the people around him. At the same time, everyone else is a stranger in his eyes:

Never in my life had I seen anyone so clearly as I saw these people; not a detail of their clothes or features escaped me. And yet I couldn’t hear them, and it was hard to believe they really existed. (chapter 1)

One could argue that Meursault fails in finding a purpose for his existence. Through his character's eyes, Camus explores the topics of existentialism and the philosophy of the absurd. From these perspectives, life itself is inherently senseless.
https://www.britannica.com/topic/The-Stranger-novel-by-Camus

The significance of the title of Camus's absurdist novel, The Stranger, is most meaningful in an examination of its irony. Though Camus's protagonist, Meursault, is dangerously odd and murderous, many readers will find that they have more in common with this sociopathic character than perhaps is comfortable. The similarities that exist between such a man as Meursault and a typical reader means that the stranger of the title is actually not a stranger at all.
Camus is famous for his philosophy of the absurd, which means that he and his characters like Meursault find the process of searching for meaning completely pointless. Because life is so bizarre and so full of incomprehensible nonsense, all citizens of the world are strangers in their own lands. In fact, interestingly, the title of Albert Camus's absurdist novel L'Étranger, in French, and known as The Stranger in English, can also be correctly translated into "The Foreigner." The disorienting sensation of feeling like a foreigner in one's own familiar space is one that is likely familiar to both Camus and his creation, Meursault.

The title refers to the central character, Meursault, who is a "stranger" in the sense that he is psychologically alienated from other people and from the world at large. Meursault does not react to events and to his environment in a "normal" way, for he feels disconnected from life. His mother's death, for instance, does not cause the ordinary grief one would expect. On the first page, he even seems uncertain of the day of his mother's death. Later, he shoots the Arab man for no apparent reason and does so indifferently, as if the act of killing someone is of no more importance than, say, lighting a cigarette or any other neutral or insignificant action. He's a stranger in his milieu almost as an alien from another planet transported to earth might be.
Paradoxically, Camus sets up the character of Meursault as an emblem of humanity in general. Though Meursault is seemingly different from everyone else, detached and alienated, he's a symbol of modern man alone in an indifferent or hostile universe. In the twentieth century, the old values, like religion, "honor," and love of country, are viewed as no longer compelling or fulfilling to many intellectuals and others. Camus's "message" can be seen as an injunction to man to create his own values in a world where inherent meaning is lacking. It is only on the final page of The Stranger that Meursault seems to grasp this, as he states that, at his coming execution, he wishes there will be a huge crowd that will address him with howls of hatred. In committing murder and thus destroying his own life as well, Meursault welcomes his punishment for an act committed in violation of the values that man himself has created.