从不 (cóng bù) means 'never' and emphasizes that an action has never happened habitually or as a general rule (present/future). It differs from 从未 (cóng wèi), which is more formal and refers specifically to the past ('never before'), and from 从来 (cóng lái), which can be used in both affirmative and negative contexts with a broader meaning of 'always/never all along'. 从不 is a fixed phrase combining 从 (from) with 不 (not).