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).
This road is very dangerous, so please under no circumstances leave the group.
By no means, can I say that it does not hurt.
This ride is by no means dangerous.
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.
決（けっ）して may also sometimes be paired with other structures that are not specifically negative, the primary example of this being ものか
(a grammar point that we will learn about later). ものか
strongly confirms a statement as being something that does not exist/will not be done.
There is no way I will hand over this treasure.
There is no way I will lend you money.