Quick answer
Nearly a tautology, and deliberately so. The verse says a witness is defined by exactly one thing.
Read both lines
The flatness is the point. There is no cleverness available here, no middle category, and no version of a faithful witness who shades things. Proverbs returns to courtroom testimony more than almost any other subject, because a community that cannot rely on sworn speech has no way to protect anyone, which is why the false witness makes the LORD's own list at 6:19. The verse will be developed twenty verses later, where a true witness is said to save lives.
Related: Proverbs 14:25 · Proverbs 6:19 · Proverbs 12:17
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.