There are three main types of narrators used in most stories (novels or short stories). First person narrators tell their story using the pronoun "I." Third person limited narrators use pronouns such as "he," "she," and/or "they." The third person limited narrator knows everything about the main character (or the protagonist), but is limited in their knowledge regarding other characters.
Guy de Maupassant's short story, "The Necklace," is written using a third person omniscient narrator. A third person omniscient narrator uses the same pronouns as the third person limited (he/she/they), but knows everything about all characters (instead of being limited to only knowing the protagonist).
The story begins with the narrator describing Madame Loisel.
She was one of those pretty and charming girls born, as though fate had blundered over her, into a family of artisans. She had no marriage portion, no expectations, no means of getting known, understood, loved, and wedded by a man of wealth and distinction.
The use of the pronoun "she" shows the narrator to be telling the story of another person (who the narrator knows well--given the in-depth descriptions). The omniscient narrator tells the story as it is, without assuming any information.
Therefore, the narrator of the story is unknown based upon the fact that he or she is not named. In "The Necklace," the narrator is simply telling the unfortunate story of the Loisels.