What is the "honest but reasonable belief doctrine"?

The "honest-but-unreasonable belief doctrine, which mitigates the culpability of a homicide, derives directly from traditional common law doctrines of self-defense and defense of others which completely justify certain homicides if the defendant's belief is both honest and reasonable. In order to justify a homicide under these traditional principles, the defendant must have reason to believe that the danger is imminent and that lethal force is necessary to prevent death or great bodily injury." (People v. Uriarte (1990) 223 Cal.App.3d 192, 197.)