Quick answer
The right words at the right moment are a made object, valuable and finished. The verse treats good speech as craftsmanship rather than luck.
Read both lines
Every element is deliberate. Gold set into silver is the more precious metal displayed by the lesser one, and the arrangement is the work of an artisan, so the image is not a lump of treasure but something composed. That is what the saying claims about speech: the value is in the fit.
The Hebrew behind "fitly spoken" is rare and its precise sense is debated, but the direction is not, since it describes a word spoken upon its proper occasion. Timing is the whole of it. The book says the same thing plainly at 15:23, where a word in due season is called good, and this verse says it beautifully instead. The one thing it will not let you do is separate content from moment. A true sentence delivered at the wrong hour is not this.
Related: Proverbs 15:23 · Proverbs 25:20 · Proverbs 16:24 · Proverbs 12:25
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.