Stated again, in different words, at verse 23. The Hebrew is literally "a stone and a stone, an ephah and an ephah."
Quick answer
Two sets of scales, one for buying and one for selling. The verse calls rigged commerce something God finds detestable.
Read both lines
The Hebrew names the fraud by describing the equipment: a stone and a stone, an ephah and an ephah, which is how the cheat actually worked. A merchant kept a heavy weight for when he was buying and a light one for when he was selling, and both looked entirely ordinary sitting in the bag. That is what makes this a wisdom verse rather than a legal one. The theft is invisible, distributed across hundreds of small transactions, and every victim is someone who trusted the equipment. The strongest word in the book's vocabulary lands on it, and the chapter thinks it worth repeating thirteen verses later.
Related: Proverbs 20:23 · Proverbs 11:1 · Proverbs 16:11 · Deuteronomy 25:13–15
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.