可不 is a fixed colloquial phrase, short for 可不是吗, used to express strong agreement with what someone just said — equivalent to 'you're absolutely right', 'isn't that so?', or 'exactly'. It often stands alone as a reply and is common in everyday conversation. The tone is upbeat and emphatic. Note: the full pronunciation is kě bù, but the input pinyin is given as kě (likely an abbreviation); in speech you always say kě bù.