Bunkai means "analysis". Literally, the kanji mean, to pull apart and look at. You might say study.
More or less always, when Karate-ka talk about bunkai, they mean the application of a kata movement. Oft-times, the bunkai they receive is less than useful for self defence, one of the original aims of the art.
The term that one might prefer to use for application is "oyo". The kanji for oyo show the terms "use" and "apply". You might say what the move is used for.
There are some good people showing good bunkai, these days. But what makes good bunkai?
For the application to be valid, it must follow this very simple formula:serves a purposeleaves the karate-ka in a better situation than they would be in if they didn't use the moveworks at a realistic distanceworks against a person attacking with a common type of attackcan be initiated without much digital dexterity (as you won't have any when under attack)has variable levels of success, so that it can be used for soft compliance or hard finishesdoes not rely upon a particular combination being thrown, but rather on pre-determined responses and pain withdrawal reflexes.