This is one of five places Proverbs warns against standing surety, and the NKJV footnotes both key terms.
Quick answer
Guarantee a stranger's debt and you will pay for it. The book says this repeatedly, and it means the pledge rather than the generosity.
Read both lines
The NKJV footnotes "surety" as guaranty and the second phrase as literally those who strike hands, the handshake that made a pledge binding. The same two elements appear at 6:1–5, 17:18, 20:16, and 22:26–27, always with the same alarm, which makes this one of the book's most insistent practical warnings. The distinction worth keeping is the one chapter 6 made: generosity gives away what it has, while surety promises what it may not have against a risk it does not control. Proverbs is enthusiastic about the first and consistently alarmed about the second.
Related: Proverbs 6:1–5 · Proverbs 17:18 · Proverbs 22:26–27 · Proverbs 20:16
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.