決（けっ）して by itself is the て form of the verb 決（けっ）する ‘to determine’. However 決（けっ）して is special in that it is considered to be a stand-alone 副詞（ふくし） (adverb). As an adverb, it carries the meaning of ‘decidedly’, or ‘assuredly’, and is used in sentences that contain ない, in order to convey ‘never (A)’, ‘under no circumstances (A)’ or ‘by no means (A)’.
Like many other adverbs in Japanese, 決（けっ）して can appear at the beginning of a sentence, or directly before the word it is modifying (which will be in its ない form).

• この道（みち）はものすごく危（あぶ）ないので、決（けっ）してグループから離（はな）れないで下（くだ）さい。
  This road is very dangerous, so please under no circumstances leave the group.
• 決（けっ）して痛（いた）くないとは言（い）えないです。
  By no means, can I say that it does not hurt.
• この乗（の）り物（もの）は決（けっ）して危険（きけん）ではないです。
  This ride is by no means dangerous.
• この子（こ）は決（けっ）して悪（わる）い人（ひと）ではないけど、時々（ときどき）失礼（しつれい）なときがある。
  By no means do I think that this person is bad, but they are rude at times.

While the translation is ‘never’, the literal meaning of 決（けっ）して is ‘decidedly not (A)’, or ‘assuredly not (A)’, simply emphasizing the negative point within a sentence.