Quick answer
Talk enough and sin will be in there somewhere: the more we speak, the more opportunities we create for exaggeration, disclosure, judgment, and harm. Restraint is wisdom because it closes most of those doors before they open.
Read both lines
The first line is an observation, not arithmetic. It does not say the talkative person intends sin, only that in a multitude of words sin "is not lacking": it tends to be in there somewhere. Exaggeration slips in, then a confidence that was not yours to share, then a judgment you had no standing to make. Nobody plans these. Volume invites them.
So the second line commends restraint, and the word suggests a deliberate holding back, reining in what could have been said. This is not the silence of having nothing to say; it is the discipline of having plenty and releasing little. Proverbs returns to this constantly (13:3, 17:27–28, 21:23) because the sages had noticed where the accidents happen. Every word is a moving part, and wisdom runs with fewer moving parts. James inherited this verse's whole worldview: "If anyone does not stumble in word, he is a perfect man" (James 3:2).
Related: Proverbs 13:3 · Proverbs 17:27–28 · James 3:2
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.