可不是 (kě bú shi) is a colloquial, fixed phrase used to emphatically agree with what someone just said — like 'That's exactly it!' or 'Isn't that the truth!'. It is used as a standalone response, unlike 就是 (jiù shì) which can also mean 'exactly' but is more flexible (e.g., as an adverb or conjunction). 可不是 is stronger and more conversational.