N3
決して〜ない
けっして〜ない
Never, By no means
この子は決して悪い人ではないけど、時々失礼なときがある。
This kid is by no means a bad person, but they can be rude at times.
Explanation
「決して～ない」 means 'never' or 'by no means'. A negative must follow. Two uses: the speaker states a firm will, swearing never to break a principle: 「決して諦めない」 (I will never give up); or earnestly tells someone never to do something: 「決して他人には言わないでください」 (Please never tell anyone). The tone is solemn and decisive.
Mixed up with
「全然 / まったく～ない」 stresses that the degree or amount is zero (not at all). 「決して～ない」 stresses firm will or strict principle, mostly about matters of principle.
In listening, when a speaker swears or gives a safety warning, 「決して」 often opens the sentence and marks a core principle.