An unreliable narrator is one we cannot trust completely for various reasons. First of all, this type of a narrator may say one thing at one point, and then something completely different at another, which makes us not trust his words. An unreliable narrator usually shows weird behavior towards themselves and others, which is the case with Montresor. He is treating Fortunado quite awfully, locking him inside a vault due to his mental instability.
When the author, Edgar Allan Poe, uses the technique of the unreliable narrator to portray his protagonist, he is only emphasizing his mental instability and giving us reasons not to trust the narrator and to question his every move.
I hope this helps! :D