Quick answer
Kind words are medicine twice over: sweetness to the soul and health to the bones. The proverb takes the healing power of a good sentence seriously enough to prescribe it.
Read both lines
The image is the richest thing in the ancient pantry, and the effects file in both departments, soul and bones, the same body-deep reach this book gave to trust (3:8) and a merry heart (17:22). The right word at the right moment does something physiological: shoulders drop, breath returns, the day becomes livable. Pleasant words here are not flattery, which this book despises, but words fitted to the hearer's need, true and kind at once. The unspoken question follows you out of the verse: whose bones have your words been health to lately?
Related: Proverbs 12:18 · Proverbs 25:11 · Ephesians 4:29
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.