means that he thinks he knows everything, then as a noun it means that he is a 'know it all'.
A little knowledge is a dangerous thing is a comment on a situation which may come about due to the influence of a know it all.
A man may go for a long trek knowing that he can survive without food for thirty days, but he dies because he didn't know that without water a person can only survive for ten days. He was a know it all. Someone would comment "Ah! A little knowledge is a dangerous thing.