Quick answer
Testimony is the setting. What comes out of a witness's mouth either serves justice or corrupts it, with nothing in between.
Read both lines
The courtroom frame is what gives this verse its weight. A false witness does not merely tell a lie; he damages a proceeding meant to protect people, which is why the offense appears in the LORD's own list at 6:19. The compression of the second line is deliberate, dropping the verb so the phrase lands as a verdict: a false witness, deceit. The verse assumes something a community depends on and can lose, which is that when a person speaks under obligation, what they say corresponds to what happened.
Related: Proverbs 6:19 · Proverbs 14:5 · Proverbs 19:5
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.