Quick answer
Knowing something is not the same as saying it. The prudent man holds what he knows; the fool broadcasts what he does not.
Read both lines
The first line is not about secrecy but about restraint, the decision not to display everything you have. Set against the second line, the contrast becomes about volume relative to content: one man has knowledge and says little, the other has foolishness and announces it. The book has drawn this before at 11:12 and 12:16, and it keeps landing on the same practical point. What most reveals a person is not what they know but what they cannot keep from saying.
Related: Proverbs 11:12 · Proverbs 17:28 · Proverbs 13:16
New King James Version®, Copyright © 1982, Thomas Nelson. All rights reserved. Full copyright information. Scripture provided by API.Bible.
Go Deeper
If this verse has been used to put weight on you that it was never carrying, ask about it directly.