It is an antiquated form of verbal negation. Saying "he knows not" is basically the same as saying "he does not know". I wouldn't call it 'archaic' because, although not used in modern, informal speech, "know not" is occasionally still used - usually for poetic reasons, when quoting famous texts, or as part of some well-known idioms. For example, two quotations from the King James Bible (originally written in early modern English) have become quite well known idioms - "they know not of what they speak", and "judge not, lest ye be judged".
However there are other structures where it is still used and not entirely uncommon - for example, we say "I have not..." just as much as "I don't have...", and some use the expression "haves and have nots" to refer to the wealthy and the poor.