An unreliable narrator is a first-person narrator with a compromised viewpoint
Narrators serve as filters for stories. What narrators do not know or experience cannot be shown to the reader. The first-person narrator is powerful because that viewpoint is the only one that we have to judge the events on the page. The reader believes that the narrator will be truthful and provide an accurate account of the story.
When we have an unreliable narrator, the reader cannot trust his or her version of the story.
These narrators may be insane, angry, strung out on drugs or alcohol, naive, foreign, criminals, liars or simply younger than everybody else.
They can be comical or absurd, tragic or serious, terrifying or surreal.