Quick answer
Watch what you say and you keep your life. The verse treats speech as the most dangerous thing most people handle daily.
Read both lines
Both guarding verbs from the early chapters appear here in one line: the word used at 4:23 for guarding the heart, and the word used at 7:5 for keeping. What they guard now is a mouth, and what that guarding preserves is nephesh, a person's own life. The second line's image is physical, a man opening his lips wide, and the picture of a gaping mouth against a guarded one is the whole argument. The book is not recommending silence. It is observing that most of what wrecks a life came out of a person rather than happening to them.
Related: Proverbs 4:23 · Proverbs 21:23 · Proverbs 10:19
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.