Quick answer
Words can nourish a person or fracture one. The NKJV's footnote is the key: a "wholesome" tongue is literally a healing tongue, and healing speech is life-giving the way a tree is.
Read both lines
The first line reaches for the book's highest compliment. "Tree of life" is what Proverbs calls wisdom herself (3:18), and here the same honor goes to ordinary healing speech, which puts a kind word in surprisingly exalted company. The second line names the damage with equal seriousness: twisted speech "breaks the spirit," and a broken spirit in this book is not a mood but a fracture at the center of a person. The asymmetry is the sober part. Trees grow slowly and breaks happen fast, so the verse quietly warns that the tongue can destroy in an afternoon what it took years to cultivate.
Related: Proverbs 3:18 · Proverbs 12:18 · Proverbs 18:21
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.