A narrator is the storyteller in a book or movie. One of the most famous literary narrators is Herman Melville's Ishmael, who tells the story of the Moby Dick.
The narrator is the person who tells the story — in other words, she narrates it. In a fictional work, the narrator is a character who relays the story from her own perspective, which is different from the writer. If you don't trust the narrator's version of the story, you may have encountered an "unreliable narrator." The Latin root is narrare, "to tell or relate," or literally "to make acquainted with," from gnarus, "knowing."