Being called benighted is much like being called naive. It means lacking in knowledge or understanding—the kind you might have if you were older or more sophisticated.
Although it sounds a lot like "being knighted", benighted has nothing to do with knights and, in fact, includes the word "night" (as in the opposite of day) and not "knight" (as in "of the Round Table"). One way to remember what benighted means is to think of a person "being nighted" or, put into the dark. A benighted person is in the dark about things: they don't know because they can't see. The Dark Ages are thought to have been a benighted time, full of primitive ideas.