An omniscient or third-person narrator is a voice that tells the story and that knows everything that happens in the story, beyond the knowledge of the characters. For example, when Kino and Juana bring their baby, Coyotito, to the doctor for his scorpion bite, the narrator tells the reader about the doctor: "The doctor had once for a short time been a part of the great world and his whole subsequent life was memory and longing for France" (page 12). This background about the doctor is something only an omniscient, or all-knowing, narrator would know--as it is not necessarily information that the characters who meet the doctor have.
The narrator also knows how news flows through the town before Kino and Juana do. For example, the narrator says:
"Before Kino and Juana and the other fishers had come to Kino's brush house, the nerves of the town were pulsing and vibrating with the news - Kino had found the Pearl of the World" (page 21).
The narrator has access to the way people are behaving about Kino's discovery of the pearl even before the characters do, as the narrator knows things beyond what the characters know. Many parts of the story are told by the omniscient narrator.