An unreliable narrator means you can't take all of the written words at face value. In other words, the narrator "lies" to the reader. The lies will often be about relatively small, but important details in the narrative. The deviations from the truth aren't always malicious.
Generally, the reader is made aware that the narrative is unreliable in some way (though sometimes this is done incredibly subtly). Some novels with unreliable narrators will eventually tell you what really happened. Many do not, at least not entirely, forcing you to make up your own mind about the truth.
Due to its nature, an unreliable narrator most often happens in first-person writing.
Here are a couple of examples:
- young, childish, naive, or simple narrator - Due to lack of maturity the narrator has an overly simple worldview. The narrator will tell things as they believe the world works. But that isn't always how things really happened.
example: Forest Gump. It is highly unlikely that Forest was as intimately involved with all those historical events as he makes you believe. But Forest truly believes he is telling nothing but the honest truth.
- evasive narrator - The narrator knows the truth but deliberately avoids talking about it, or subtly alters it. There is no maliciousness involved. The narrator is deluding you with the best intentions.
example: Life of Pi. Pi tells a fantastical fairy tale with cute fluffy animals. Only at the very end does Pi reveal that the cute animals might be a substitute for a far more harrowing truth.
- self-delusive narrator - A variant of the evasive narrator. The narrator is at least subconsciously aware of the truth but does not want to admit that truth to himself, let alone the reader.
- deliberate liar - The narrator lies to you. Deliberately. Maliciously. Some classic tropes here are the villain pretending to be the hero, or a psychopath narrator.
example: Gone Girl. The viewpoint switches in the second half of the novel. Certain events are retold, but entirely different. Which of the two narrators is lying? Or are they both lying?