This saying prepares the ground for 18:21; the two are read together as the book's fullest statement on the harvest of speech.
Quick answer
You eat your own words: what the mouth produces comes back to the table as a life's actual diet, for better or worse.
Read both lines
Both lines run harvest language for words through the same startling loop: the eater of the crop is the speaker himself. The verse is carefully neutral, and the neutrality is the message: it does not say the harvest will be sweet, it says the harvest will be yours. Words go out into marriages, friendships, and reputations, and years later their produce returns as the atmosphere you live in: trust or wariness, warmth or distance. Speech is farming, seed now, food later, and no farmer gets to disown his own crop. The next verse raises the stakes to their true height.
Related: Proverbs 12:14 · Proverbs 13:2 · Galatians 6:7
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.