Bunkai (分解) literally means 'analysis'. We are analysing the kata - using kata sequences and techniques to respond to an pre defined attack from a partner.
"Karate-do is a noble martial art, and the reader can rest assured that those who take pride in breaking boards or smashing tiles, or who boast of being able to perform outlandish feats like stripping flesh or plucking out ribs, really know nothing about karate. They are playing around in the leaves and branches of a great tree, without the slightest concept of the trunk." ~ Ginchin Funakoshi Karate-Do Nyumon