Quick answer
In a wise person, the heart runs the mouth, not the reverse: what is stored inside instructs what gets said.
Read both lines
Speech, in this book's anatomy, is downstream; the mouth is a spout, and the vessel decides what pours. The verse explains the difference between two kinds of talkers: the one whose words arrive from somewhere, weighted with thought, and the one whose mouth publishes first drafts on arrival. Jesus pressed the same anatomy to its edge: "out of the abundance of the heart the mouth speaks" (Matthew 12:34). The practical direction is the kindness: better speech is not primarily a mouth project. Fill the heart, and the mouth finds itself taught.
Related: Proverbs 15:28 · Matthew 12:34–35 · James 3:2
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.