What is mean by wise is he who knows that he does not know?

There is more information available than a single person can know, even about a very small and focused area of expertise. A wise person recognizes that there is information he does not know, and based upon this knowledge seeks out others who have that information and knowledge.

Foolish people tend to believe, or at least spout the opinion, that they know everything they need to know - they tend to not seek additional information or knowledge and are limited to whatever information they already have.