Quick answer
A righteous person's words assay as refined silver; a wicked person's heart assays as nearly nothing. The mismatch of body parts is the point: speech reveals what the heart holds.
Read both lines
The proverb puts two things on the appraiser's scale, and the surprise is that they are not the same organ. The righteous man's tongue comes back "choice silver," refined, the impurities burned off; the wicked man is weighed at the heart, the source itself, and comes back "worth little." The asymmetry is deliberate: the tongue is the heart's outlet, so you can assay the source by sampling the stream. If the tongue runs silver, there is a treasury behind it; if the heart is nearly worthless, no eloquence downstream can add value the source never held. Jesus pressed the same logic: "out of the abundance of the heart the mouth speaks" (Matthew 12:34).
Related: Proverbs 25:11–12 · Matthew 12:34–35
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.