Quick answer
Where understanding lives in a person, wisdom shows up in their speech. Where understanding is absent, correction arrives from outside instead, the hard way. The verse contrasts two teachers: your own understanding, or consequences.
Read both lines
The first line traces a flow: understanding in the heart surfaces as wisdom on the lips. Speech is where the interior life becomes audible, a theme this chapter keeps circling.
The second line is blunt in the manner of its time. In its ancient setting the rod almost certainly evokes actual physical discipline or judicial punishment, not merely an abstract consequence, and the proverb records that severity without flinching. The deliberate harshness is part of the teaching. From there the transferable wisdom is easy to recognize: the person who refuses instruction from within will be instructed from without, and reality teaches painfully what humility could have learned gently. Understanding is cheaper tuition.
Related: Proverbs 26:3 · Proverbs 17:10
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.