Wuthering Heights has two main narrators: Lockwood and Ellen "Nelly" Dean. The primary narrator is Lockwood, who begins and ends the narrative and is recording the story that he hears from Nelly. Nelly is Lockwood's inside source of information, though, as he can only directly report what he witnesses in the present time – beginning in 1801, the year before Heathcliff dies. So, Nelly is telling Lockwood her version of the events, which then get filtered and recorded through his perspective. In cases where Nelly was not a witness to the events, she fills in the story with either someone else's eyewitness report to her, or she quotes a letter.
It's important to remember that both Nelly and Lockwood have their own interests, biases, likes and dislikes, so what we read is a highly biased account of the story of the Linton, Earnshaw, and Heathcliff families. With the exception of a few stretches in the novel, we are always receiving information through the double lens of these two characters, neither of whom is objective or detached.
Brontë provides a few hints that our narrators have their own plans, desires, and interpretations. An example: Remember how Lockwood grossly misjudges Heathcliff on their first encounter? He writes, "Mr. Heathcliff and I are such a suitable pair to divide the desolation between us. A capital fellow!" (1.1). We realize pretty quickly that Heathcliff is a lot of things, but a "capital fellow" isn't one of them, and it takes Lockwood a while to get the hint. Plus, remember how he thought he had a chance with Catherine Heathcliff? Right.
OK, now just one (of many) examples of a problem with Nelly's narrative. She admits that she tells the story "in true gossip's fashion" (8.87). How Nelly sees herself is important because it tells us about the kind of narrator she will be. As she tells Lockwood:
I certainly esteem myself a steady, reasonable kind of body . . . not exactly from living among the hills and seeing one set of faces, and one series of actions . . . but I have undergone sharp discipline which has taught me wisdom; and then, I have read more books than you would fancy, Mr. Lockwood. You could not open a book in this library that I have not looked into. (8.86)
Nelly wants Lockwood to know that she's not just some uneducated servant. But any time you have narrator who reads a lot, it's a red flag. By adding this detail, Brontë suggests that Nelly likes stories, or fiction, and possibly gets some of her ideas from books. We can tell she likes drama and symbolic detail, as when she tells the story of Heathcliff putting a piece of his hair in Catherine's locket.
While Nelly's story involves less speculation than Lockwood's, her advice to young Heathcliff reveals an active imagination, the result of reading perhaps one too many Romantic novels:
Who knows but your father was Emperor of China, and your mother an Indian queen, each of them able to buy up, with one week's income, Wuthering Heights and Thrushcross Grange together? [. . .] Were I in your place, I would frame high notions of my birth. (7.44)
The point is, Nelly adds her own creative interpretations to a scene she is too much a part of to describe objectively. The story that Nelly narrates takes place, for the most part, in the past, and the characters cannot refute her version of the facts. Her tendency to romanticize makes her a compelling but unreliable narrator.