In "The Cask of Amontillado, what does, "A wrong is unredressed when retribution overtakes its redresser" mean?

"Redress" means "to set right" or "to rectify"

"Unredressed"--- the opposite of above.

"Retribution" means "punishment that one deserves for a wrong he has committed"

Montressor is saying that Fortunato has not "paid" for having wronged him, so Montressor is justified in taking restitution for the "insult" done to him.