Cannot is a modal verb that shows no ability or permission to do something. It means something is impossible or not allowed. It is the negative form of 'can'.
I cannot swim very well.
You cannot enter the room now.
Sometimes, especially in older texts, 'can not' is used as two words to emphasize refusal or prohibition. Today, it is mostly written as 'cannot' or 'can't'.
You can not ignore the rules.
He can not refuse such an offer.
