Quick answer
The fool's mouth carries a stick. The wise man's mouth carries a guard, and the verse compares what each one is armed with.
Read both lines
"A rod of pride" is an odd and vivid phrase, and the image is of a man swinging something. What he swings is his own arrogance, and the shape of the proverb suggests it comes back around, since a rod in this book is usually applied to the person who needs correcting. Against that, the wise man's lips preserve him, using a guarding verb, so the same organ that arms one man defends the other. The chapter will return to this at verse 23 and again at verse 25: speech in Proverbs is never neutral equipment.
Related: Proverbs 12:6 · Proverbs 13:3 · Proverbs 18:6–7
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.